This is just my own opinion and not a critique of your idea, but if I was going to cover a large part of my cham's cage with something I would prefer getting a large laminated background print of tropical foliage or forest instead of a rock wall. After all, chams are not terrestrial or rock dwellers, they are arboreal and the forest or plant backdrop would seem to suit them better. Also, the amount of cage space a fake rock background would fill could be better used with more foliage. I've done some searching around online for tropical forest images (there are LOTS!), downloaded a favorite, and emailed it to a copying service that can laminate prints to your desired size. Then I taped the image on the outside of my cage mesh so it didn't take up any interior space. I don't have a picture of one cage I did this for but it was really really great looking. I've also done this for my Malaysian leaf frog terrariums...giving them a leaf covered forest floor scene that also blocked disturbance.
Oh, and for my melleri room I bought one of those large wall sized photo posters of a forest and used it as background for the free range area. Looked fantastic and just changed the entire mood of the room. But one of the chams kept trying to reach the "trees" in the image and I'd find him perched on the very tip of a real tree as close to the wall as he could get. Some of my visitors gave me odd looks though, but being a cham addict I was used to all that.