Lotta cool options. Most of these options aren't persistent though, correct? As in, I can't set up a nice laboratory and have it still exist upon reopening the space.
That's an interesting idea, Zagnabbit. You wouldn't happen to have the stats on such a beast, would ye?
No stats no.It's basically a minor artifact that is a permanent, mobile gateway to a small, personal Demi-Plane.
Pathfinder #24 "TheFinal Wish", secondary article "Decanter of Black Death"
The above article does stat that the creator made about 300 of them though, so they aren't exactly a unique or campaign breaking item.
Any permanent, inhabitable, extradimensional space that is large enough to contain several people is going to be prohibitively expensive. The ability to use it as a treasure cache is just too much in game terms (since you never need to worry about weight or storage again, plus if it's mobile you can always have every piece of mundane equipment you could ever need). A "familiar pocket" X100 maybe?
I have a home-brewed item that does just that plus some other things, it's a Staff and it's insanely expensive even with my half-assed pricing making it barely affordable for a big level player and it only provides a space that is about 20,000 cubic feet. (like a 3 bedroom house as an example).
Talk to your DM about this, if he doesn't mind a mobile spell lab it's not an unlikely get item. While it's gold cost would be staggering it can be built in such a way as to be non-game changing, beyond saving the party from nighttime random encounter tables.