Summary
Cargo in the dedicated cargo hold of vessels with that feature have special encumbrance rules:
For listed equipment, there is a 10x packing rule (which I've seen proposed on various forums) for the dedicated cargo bay of light freighters.
The goods stored in the cargo hold are difficult to access at all (possibly requiring skill checks if the vehicle is moving), being safely locked in place and whatnot; in addition, if someone actually gets at the cargo they (or the ship itself) will be in danger due to the cargo moving around when the ship manoeuvres.
For a qualitative (narrative) description of the cargo capacity: it is about 1 cargo crate per 5pts of cargo encumbrance.
For a mass limit, one can use resources like wookiepedia,
this may be used to limit the number of crates of things like heavy machinery,
or raw materials.
The same encumbrance limit number constrains the amount of extra personal equipment that the crew/passengers can pack into the living portions of the ship. The 2x packing rule would apply, and this equipment would be relatively easy to access even when the vehicle is in motion (unlike the cargo in the dedicated cargo hold).
This last bullet implies that there is a separate "cargo hold" capacity from the "living quarters" storage capacity, and that are covered by different packing rules.
Finally, these rules only apply to vehicles designed for cargo, so, for example, they don't affect the capacity for a TIE fighter. However, in my quick check, they do seem to make sense for some of the land transport vehicles (that heavy truck like ground vehicle whose name I can't seem to remember), but wouldn't for a swoop.
Discussion for YT-1300
I'll use the holdout blaster as a representative item. If it's similar to a current day handgun, it fits inside something like a 10cm by 30cm by 50cm box,
and has a mass of about a kilogram (including box). Thus about 66 of them fit inside a cubic meter.
If we go by the rules as written a YT-1300 can carry 330 such pistols (packed) -- this works out to a volume of about 5.0 cubic meters, and a mass of 330kg. This is about the cargo capacity of a large cargo van.
Note that this is one way to resolve the apparent conflict: Han and Chewie are "two guys with a van" instead of having a vehicle that is capable of carrying industrially significant quantities of cargo. This approach leaves aside the issue that external sources rate the cargo capacities in 10s of tons for this type of ship, and it scales up to 1000s of tons for larger ships, which would not be reflected in these rules.
Using the 10x packing rule allows a YT-1300 to carry 1650 holdout blasters; this works out to a volume of about 25 cubic meters, and a mass of 1.65 tons.
This compares favourably with the 33 crates obtained by diving the encumbrance rating by 5 and assuming that each "crate" is slightly more than 1 cubic meter (especially considering that there is always wasted space in trying to pack cargo). Going the other way makes sense too: 1650 pistols in 33 crates works out to 50 guns per crate. Note that a volume of 33 cubic meters is about the size of a 20ft shipping container, so a YT-1300 would be somewhat like a large diesel truck or smallish tractor trailer. Note that heavy machinery or bulk metals could easily exceed densities of 100kg/m^3 and in those cases the mass constraint could come into play.
For GR-75
By the rules, a GR-75 has about 4.8x the capacity of a YT-1300, so something
like 160 crates. If you consider the underside of this transport you can estimate that that it has something like a few hundred (at most) containers attached to it -- so it is line with the qualitative "one crate per 5pts of encumbrance" rule. One can further scale up the effective capacity by ruling that for "medium freighters" a "crate" is larger (maybe 2-3 cubic meters?) and so on.
This means that it might be worth considering a 20x or more "packing rule" for listed equipment when packed into the cargo bay of a medium freighter. This kind of logic could scale up to heavy freighters and so on, but I haven't worked through many examples since so far, I've only worried about light freighters.
It's weirdly difficult, probably because it isn't something the designers thought anyone would particularly want to do.
Your best bet is probably a carefully worded Wish, and the hope that this being an extremely low-power use of Wish the DM won't twist it on you.
However, there is one explicit way - be a 14th level Wizard of the Transmutation school. Then, spend 8 hours creating a transmuter's stone, and another 10 minutes using the Major Transformation ability. This will allow you to turn one object no larger than a 5-foot cube into another object of similar size and mass and equal or lesser value.
And yes, this is ridiculously restrictive. And it's bizarre that this appears to be the only way, which is why I suspect this isn't something the creators of the game thought anyone would care about.
Even more restricted alternatives include:
- Fabricate, which allows you to turn raw materials into finished objects.
- Stone Shape, which allows you to turn something made of stone into something else made of stone.
For the specific case of making pumpkin carriages, there's a spell that should work for you - Creation. You'll need to cast it out of a fairly high-level slot to create a full-sized carriage, but if True Polymorph is on the menu this shouldn't be a problem. Pumpkin is quite definitely vegetable matter, so it should last a day; plenty of time to get Cinderella to and from the ball. Of course, 100gp would get you a mundane, ordinary carriage, so you could just do that. Alternatively, Wish can create pretty much any nonmagical object, so you could do that too.
Best Answer
There are no official mechanics given for this in 5e; only the results of the process are briefly mentioned.
The process of elemental binding is mentioned a number of times in the Wayfinder's Guide to Eberron... but no rules are described for how this is done.
Lightning rails and elemental airships are briefly mentioned in Chapter 1, under the heading "A Magical World" (p. 6):
There's barely more detail about them (and elemental galleons) in the "Transportation" section of Chapter 2 (p. 46), but no mechanics are provided on how they are created.
Elemental binding is momentarily mentioned again in the description of the nation of Zilargo, earlier in the same chapter (p. 38, italics mine):
In Chapter 3, the same nation comes up again in relation to elemental binding; the "Gnomes of Zilargo" section has a "Zil Schemes" table, and the first row provides a relevant character hook:
Khyber dragonshards are described in more detail in Chapter 5 (p. 113):
A later part of the same chapter mentions a relevant magic item, the wheel of wind and water (p. 117):
Chapter 6 is about Sharn, City of Towers. It provides a number of tables for use in quickly generating a basic story set in Sharn. The table "The Hook" contains this possible plot-hook:
And finally, there's one last mention in Appendix A: Further Reading, under the "Eberron Sourcebooks" heading (p. 165, italics mine):
I don't own this book, but it is likely your best bet for finding more information on the actual process of elemental binding. Of course, any mechanics from it that you wish to implement in 5e may require some changes in the process of converting it to a different edition.
As the beginning of the section notes, the Magic of Eberron book is available as a PDF from DMsGuild.com (as are the other 3.5e/4e sourcebooks mentioned, presumably).
That's basically every mention of bound elementals in the Wayfinder's Guide to Eberron. There are no mechanics given for binding the elementals, only interacting with those already bound and what they might be bound to. It directs readers to a 3.5e book for more details.