I've seen people use rock. The one guy (sphelps) attatched a bunch of rock to the back of his tank. Kinda a cave look. I think he attached all the rock to a piece of Acrylic first and ziptied the rocks or something but you get the idea.
http://i11.photobucket.com/albums/a1...7lowertank.jpg
http://i11.photobucket.com/albums/a1...s/DSC_0018.jpg