Rules:
*You can teleport into and out of it at will
*It has a couple of plug sockets and can connect to internet from the region you teleported in from
*You can take objects and people with you
*As already stated, it is (3m)^3 (3m*3m*3m). The walls are plain plaster with a light in the middle of the ceiling. The pocket dimension is topologically toroidal, so if there weren’t walls and a ceiling/floor (which you can actually destroy) you would loop if you went more than 3m in any direction. Gravity, then, is artificial and can be altered to anywhere from 0 to 2g from a dial on the wall.
Edit: additional specifications
*You can only teleport out to where you teleported in from.
*Time proceeds at the same rate inside the pocket dimension
*There is an eject button for those inside to get out if something happens to you
Assuming that I can take it wherever, and when I’m in it, no one can see any trace of it…
I would find locations where valuable items would be in the near future. Go to that place, hop in my box, and wait for night fall. Then I would magically materialize in a strange masked bandit costume, rob them blind, and disappear. Then pop out a few days later in different clothes to make my casual exit.
Ideally, I’d go in and out inside a bathroom nearby so it’s less conspicuous.
Thievery and smuggling, perfectly designed for that.