As is, it is overpowered.
Replacing dice is a very strong feature, and the divination wizard signature.
Making this a 2nd level spell means any wizard can use it... and many will be able to abuse it.
Divination wizards can abuse it, starting level 6.
As wizards of the Divination School recover spell slots when casting divination spells, a 6th level wizard can spam this 9 times each day - and still get plenty of level 1 slots to keep casting.
It becomes even more silly at higher level ; at level 17, it means 46 uses per day, not even considering arcane recovery.
It gets worse at level 18, with Spell Mastery
Any wizard will be able to cast this at will. If you don't like the roll you just made, cast it again. You'll eventually roll high / low enough to auto-succeed any ability check, or ensure the guy you intend to charm to auto-fail.
Some solutions?
Option 1: don't homebrew a spell
Making this diviner unique feature a spell makes it available to anybody. It will feel less unique, and remove the spotlight from that subclass. You may prefer to homebrew a feat to allow more portent dice (1 or 2), or short rest recoveries (probably too much). It may be the good way to see it used more often.
Option 2: make it higher level
Bluemoon's 5th level is probably a good start - but depending on the class restrictions you have in mind, level 6 would be a good way to ensure no warlock ever gets to spam this on short rests, or diviners can't use recycled spell slots to cast it again.
Following that path, I'd advise you to change the upcasting ability to add more dice, instead of changing their results. A +3/-3 probably does not justify the use of a ninth-level slot, but 4 dice might.
The problem with this option, as pointed out by @Ben Barden, is that it removes access to the spell to low-level casters. You should probably look for other options.
Option 3: 1-round duration
If you like the idea of wizards spamming this (I do!) - limiting its duration is probably the way to go. If a PC wants to spend the entire fight predicting rolls for his allies - let him spend all his actions doing it.
If you choose this solution keeping it a 2nd level spell, you'll probably want to limit its use to combat rolls - barring ability checks, to avoid Spell Mastery out-of-combat shenanigans.
Best Answer
This spell has (almost) no mechanical benefit to balance
The only time that casting this spell would have any mechanical benefit would be when you needed an item made of steel and for some reason only had access to a glass version of that item. Given that such a glass item is at least as difficult and expensive to create or acquire as the steel version and is certainly much rarer in practice, casting this spell will likely never have any mechanical benefit. We typically evaluate "balance" by weighing the cost (in this case, a spell slot, and possibly a spell selection) against the benefit gained. But if the benefit is purely the subjective cool factor of having a glass weapon or tool that's functionally identical to an ordinary metal version, how can you balance the cost of a spell slot against that? It's certainly fine to have "flavor" spells, but without any significant mechanical benefit, there is nothing to balance, unless you have some other criterion you want to use to define balance.
A few existing spells and abilities are similar to this
For what it's worth, there are a couple of spells or other abilities with somewhat similar effects, which you might be able to use or adapt for your purposes. First, there is the druid cantrip Shillelagh, which buffs a single weapon at a time. One approach you could take is to have this spell be a cantrip that works similarly, being limited to a single glass item at a time.
Second, the wizard school of Transmutation includes the Major Transformation feature, which can (with some favorable DM rulings about relative mass and value of glass and steel objects) transmute a glass object to steel permanently. You could allow the ability to instead give the object the physical properties of steel while retaining the appearance of the original glass. While not RAW, this would be a purely flavor change to the ability as written.
There is also the warlock Pact of the Blade:
The reference to the weapons section of the PHB makes it clear that choosing the "form" of the weapon means that you choose which kind of weapon you want to manifest. But most DMs will also allow the weapon's appearance to be customized, or base it on the nature of the warlock's patron, rather than have it simply manifest as a perfectly ordinary-looking weapon of the chosen type. So a glass-looking weapon that functions the same as a steel one is certainly within the realm of possibility here, given the right DM, and the right patron.