One of the most iconic wondorous items in D&D/pathfinder is the Bag of Holding. Marry Poppings has one, as does Hermione. It's a must have magic item. It does have flaws. Size of the opening is one (half a meter), another is the lack of air. As a result a creature placed inside the bag survives for 10 minutes before suffocating. The latter problem seems to appear really frequently.
Technically a horse (large) shouldn't fit it, but people, ponies (medium) and animal companions do fit it and is a handy place to store them. So is there a workaround?
The rules say making holes in it, breaks it, so the easy option is out. So a custom item is the solution. Bag of holding is made with Secret chest and 1.25 kgp for the smaller one. Making one imbued with Air bubble seems obvious at first, but would not make a difference once the rules are read. An alternative is using Portable hole, but it's not quite the same as a bag.
There is a homebrew 3.5 Pocket tent which is like Hermione’s tent and uses not only Secret chest, but Rope trick (for some reason) and Secure shelter. The pricing is disproportionately cheap, which isn't a good idea. Secure shelter does not specify what happens what happens in a vacuum or underwater (it probably fails to be safe), but sounds like the right thing.
The rules don't give a specific price to make up items vaguely imbued with spells, but the closest is a x1.5 in price.
So a Carrycage of holding needs secret chest, secure shelter and 1875 gp for the smallest version (1 cubic meter fits a crouching person) or 3750 gp for type II (2 cubic meters is plenty of space for a person).