Below is a great step-by step piece for building a rock wall for beardies, but you can tailor it for a background/basking rock or anything you like really! I've just used it to build a rock climbing wall for the back of my Stimmie's enclosure.
I am the LEAST creative/handy person around and I had a really good time building this. The guys at the hardware store all got to know me because I kept coming in to get more materials etc. It's really cheap - just use styrofoam from the local fruit shop - buy some cheap grout and PVA and you're pretty much done! And you can make it perfectly suitable for your snake (ie I did mine with loads of crevices and overhangs for hiding - a big basking rock, and covered the final layer with desert sand for a really great texture and colour.)
I will post pics of mine soon! But in the meantime, I highly recommend these instructions!
http://www.beardeddragon.org/articles/falserock/
