These things:
cubes
You put them together yourself, and can combine to make any size and shape you need of something. (ie, the "opening" doesn't have to be one square by one square....it can be four squares by two squares...or any combo)
I have a deLUXE guinea pig mansion made out of them (as per cavycages.com). I zip-tied it in a million places for added sturdiness, and the thing is rock-solid.
I was wondering if they could be used for a small dog crate as well? Does anyone see an obvious problem with it?
ps...now that I think about it, the ones I linked look like a "mini" size, or something. I have the ones like from WalMart or Target, that are 16"x16" a piece, and like I say, obviously that's minimum size, not maximum.