I took another wonderful class at www.theocassionalartist.com with Dina Wakley http://dinastamps.typepad.com/.  We made these with plaster, painted and carved them, then covered them with wax.  At that point you could carve some more or if you didn't like what you carved in teh wax, you could melt it again and start over.  This was so much fun. I never would have done this on my own so it was great to take a class.  Thanks Dina!