Your best bet is to go to a garden supply store, or your local fish store (LFS) and buy the slate. Then you can stack it however you like. I have caves that are 4-7 inches high and make all sorts of condos. They've never fallen except in the case of my cobalt zebras deciding to wage war on one another by seeing who could dig out enough gravel to topple the other fish's cave. But if you wanted to eliminate that risk, just use some aquarium sealant to glue the pieces together.