I know that some folks have used that green floral foam to get the basic shape of their project. It's the stuff they use for doing silk flower arrangements. It comes in different sized blocks, and you can deform it easily with finger pressure, or use different cutting/carving tools. I'm sure once you had it carved, you would have to put some kind of coating over it to make it keep the shape.
I've never used this myself, so not sure how effective it would be.