The foam is cheap and so is packing tape. Give it a try and let us know if it works. Depending on ow much you allow it to expand, it wieghs about the same as styrofoam.
Cheechukranch, I'm with you. I've had great success with Great Stuff. As for the center being mushy and hard to sand, it will get as hard as the outside if you let it cure fully. Then its completely sandable.