Yeah.... bought a couple of polystyrene sheets from Clarks Rubber (just the 3cm stuff.. think its about 8$ for 1200x600 sheet) ... used some no more nails to glue it all to the back and sides , then took to it with a screw driver and my fingers just ripping out bits and pieces to give it a rocky type look.... Hit it with a heatgun for a couple of seconds to smooth down the little loose bits and pieces... Then just mixed up some cement , did 3 coats (first one is pretty watery , you can go thicker with the next coats)... once it was dry used some acrylic paint (just the cheap stuff from the $2 shop) and then sealed it all with a whole lot of PVA glue... theres a pretty good tutorial thread on here that will give you heaps more detail on making rock walls and hides