Depending on what your plans are to finish it, don't use styrofoam. Styrene in a component in most fiberglass supplies and fiberglass resin will "eat" whatever you've shaped. That is if you plan on fiberglassing whatever you've molded. It costs a little more, but the best material to use would be
"wonder foam". It's more like surfboard foam and will not disssolve if you lay fiberglass over it. If the link doesn't work you can find it at selectproducts.com. Search for "wonder foam".
HTH
Adam