We use these plastic dividers that slide into these metal slots that are attached to the sides (very hard to explain). I found them at PetLand for $9.95 each, or something around there. I hope that's counted as a good description, I threw out the bags they were in (although I planned on saving them for future reference).

Two of my new Bettas, Ocean and Aqua, live in those little condos as well, in my bigger tank (the picture above was not taken recently, so it does not show them). But I have to make sure they are right against the glass, because the smaller fish (such as glass catfish) swim in between the betta's compartment and the glass, and they get stuck.