A better foam to use is the foam insulation for walls. if you go in a home depot or something you can always ask a guy if they have broken sheets and get it free!. the stuff carves so much better you can actually carve in detail like stone walls or steps etc, around here it pink, i think in the U.S its blue
looks like this its called rigid foam insulation
http://www.daviddarling.info/images/foam_b..._insulation.jpg