You can also try pvc with gravel siliconed to it to camouflage it, can also try slate caves.. Basically rocks and aquarium safe silicone is your best friend..
I use plumbers epxoy to make caves and overhangs w/lace rock. When I set up my new tank, I will be using the epoxy to attach small black/gray PVC fititings to the backsides of the caves and walls of rock.