There is no perfect storage

Any time a creature uses an item for storage, that item will be vulnerable to destruction and loss. However, in my experience, folks—that is, PCs and NPCs—are first satisfied by a Heward's handy haversack (Dungeon Master's Guide 259) (2,000 gp; 5 lbs.) then by an enveloping pit (Magic Item Compendium 159) (3,600 gp; 0 lbs.) or six, despite these items' vulnerabilities and the possibility of losing them. These have been the go-to options over multiple campaigns that made it to the late teens with experienced players. Higher-powered storage just wasn't typically necessary. (And, when it was—for, like, important corpses and stuff—the PCs would leave whatever it was in the hands of an NPC who was either oblivious or in a fortress.)

I bring this up not to minimize your concern! My opinion as a DM is that NPCs generally don't seek to destroy PCs' stuff but to destroy the PCs and take the PCs' stuff, just like the PCs do. A DM that doesn't share this opinion may go so far as to have roaming bands of invisible knife-wielding kobolds that, for kicks, slash any sack dangling from an adventurer's belt. If the DM's running a campaign like that (probably to a lesser degree, obviously) then it may be wise to just ask, either in character or out of character, "How does a guy keep his stuff safe around here?" Have the DM (or an NPC) tell you (or your PC) the DM's expectations (i.e. how to keep your stuff safe).

Nonetheless, below are some storage ideas, assessed in light of the question.

Magic items that are extradimensional storage spaces

A bag of holding (DMG 248) (2,500+ gp; 15+ lbs.) is particularly vulnerable to destruction because "[i]f the bag is overloaded, or if sharp objects pierce it (from inside or outside), the bag ruptures and is ruined." To be clear, that means "[a]ll contents are lost forever." And, sadly, this vulnerability extends to everything that's like a bag of holding, from a belt of many pouches (Complete Arcane 147–8) (11,000 gp; 1 lb.) to a druid's satchel (Dungeon #92 103) (3,000 gp; 5 lbs.) to a Heward's handy haversack (DMG 259) (2,000 gp; 5 lbs.) to a nondimensional trunk (Arms and Equipment Guide 50) (30,000 gp; 200 lbs.) (steamer not elephant).

For the question's purposes, the best of these may be the belt of hidden pouches (Races of the Wild 173) (5,000 gp; 1 lb.) because at least it's concealed. Its capacity is comparably insignificant, though.

A portable hole (DMG 264) (20,000 gp; 0 lbs.) or its cheaper and deeper brother the enveloping pit (Magic Item Compendium 159) (3,600 gp; 0 lbs.) can be folded into a pocket square, but retrieving anything from it can be a woeful exercise, taking far longer than you want.

One spell and one class feature that are extradimensional storage spaces

The 1st-level Sor/Wiz spell hoard gullet [trans] (Dragon Magic 68) puts items the caster swallows into an extradimensional space, but the only way to retrieve anything from that extradimensional space is by barfing forth all of the items in extradimensional space, making it awesome but still impractical. The nomad alternative class feature personal space provides a more readily accessible internal storage area, but it doesn't hold much unless one is a high level nomad. (Also see this question.)

(War Story: I DMed Castle Amber converted to 3.5. The adventure frequently features thousands in coins as treasure… often in silver. Because exiting Castle Amber was impossible, and the party entered with no extradimensional containers, upon leveling up during the adventure the party wizard picked as a spell to add to his spellbook the hoard gullet spell and used it constantly on himself and his familiars so that the party could tote around their wealth.)

The problem with having your own demiplane is getting there and back

A ready drink helm (Dragon #294 80-1) (150 gp; 2 lbs.) (also see this question) allows a dude to consume as a free action a potion of plane shift, arrive at the demiplane of storage (that's actually quite small—like a 10-ft. cube—so that the plane shift effect can't deposit him 5d% miles off target), take a move action to retrieve an item from the storage demiplane, and consume as a free action a potion of greater plane shift to return exactly whence he came, the whole process leaving the dude with, on his turn, still a standard action and a swift action to employ whatever he retrieved. This is incredibly convoluted… not to mention expensive. (A potion of plane shift [conj] (PH 262) (5th-level spell at caster level 9) (2,250 gp; 0.1 lbs.) and a potion of greater plane shift [conj] (SpC 159) (7th-level spell at caster level 13) (4,550 gp; 0.1 lbs.) can be created by the prestige class master alchemist (Magic of Faeûrun 34–5) at prestige class levels 3 and 7, respectively.)

It would be better to access a demiplane through a moving portal à la the feat Create Moving Portal (Perilous Gateways Web column “Portals of Anauroch: At’ar’s Portal Collection” available here). While carrying around a hula hoop of adamantine or something may also seem impractical, it'll generally be less vulnerable than a bag of holding. However, this option, too, will be extremely expensive and isn't available at the typical magic mart.

Shrinking items has its own problems… and a horrifying solution

Casting on important nonmagical items (or magic items that have been made temporarily nonmagical with a dispel magic effect) the 3rd-level Sor/Wiz spell shrink item [trans] (Player's Handbook 279) then the 5th-level Sor/Wiz spell permanency [univ] (PH 259–60) allows the caster to change the item from its original state into a square of cloth and back, but that's expensive, complicated, and requires being that caster or having a significant Use Magic Device skill modifier.

This DM once approved for use in one campaign a glove of the master strategist (Ghostwalk 71) (3,600 gp; 0 lbs.) to which was added a second glove of the master strategist (Ghostwalk 71) (3,600 +50% = 4,400 gp; 0 lbs.) for a PC that really needed to have two things always at the ready. (He'd snap twice.) The campaign survived without issue and everyone was happy, so that's a possibility. (The glove is a better and less expensive glove of storing (DMG 257) (10,000 gp; 0 lbs.), so you know.)

I was very close to suggesting straight-up the deeply obscure dragonskin bag of Grendel (Dragon #329 26) (18,000 gp; 8 lbs.) that's otherwise like a bag of holding (type IV) except it doesn't send items put into the bag to a demiplane but actually physically shrinks items put into the bag to 1/16 their normal size. (Creatures and magical effects won't go in.) However, the text notes, if a dispel magic effect is successfully used on the bag, the contents resume normal size, destroying the bag. Thus, in this player's opinion, the dragonskin bag is even more fragile than a bag of holding. (The bag's default caster level, by the way, is a mere 5.)

That solution? An embedded warforged component

Eberron Campaign Setting says on Warforged Components, in part, says

Any character capable of creating a magic item can make the same magic item [taking the same amount of time, gp, and XP] as a warforged component, and any warforged character capable of using a magic item can use the same item as a warforged component.… A warforged component usually occupies the same space on the body that a magic item of the same kind normally would. Components that do not occupy any space on the body cost twice what they would cost as ordinary magic items. These components only work when bonded to the body of a warforged; they are not usable by members of any other race. There is no chance of damaging an embedded warforged component when the warforged rolls a 1 on a saving throw, and an embedded component cannot be targeted by an attack independently of the warforged who bears it. (268 and emphasis mine)

…Which is fine if you're a warforged, but what if you're not? In that case, for near-perfect storage, a nonwarforged creature could commission a wondrous item craftsman to create an embedded warforged component dragonskin bag of Grendel that has a market price of 36,000 gp. Then the creature could make a Use Magic Device skill check to emulate a race (DC 25) (also see this question) and, with success, for 1 hour, the embedded warforged component dragonskin bag of Grendel would function for that creature as if the creature were a warforged.

While its embedded, the embedded warforged component dragonskin bag of Grendel can't be targeted, isn't subject to vagaries of the random number generator, can't be stolen, and can't even be opened (i.e. activated) by anyone but the creature in whom it's embedded. And no pesky adventurers can use teleport or plane shift effects to visit the bag's interior. (The dragonskin bag won't let in creatures, remember?)

While this is a lot of resources to invest, I can't think of anywhere more secure that still meets the question's high standards. For example, while similar results can be had with the embedded warforged component camouflaged compartment (Dragon #352 80) (12,000 gp; 0 lbs.), the compartment remains an extradimensional space—therefore vulnerable because of it—, but it is not otherwise like a bag of holding, so it won't implode when the warforged (or ersatz warforged) enters its ally's enveloping pit. This would, I think, be the second safest—and perhaps less embarrassing—alternative to the embedded warforged component dragonskin bag of Grendel.