Don't forget cardboard, or card stock. Simple sheets of cardboard are excellent for making templates before you start hacking away at metal. Also, it can serve as a good base for your Bondo work, etc. Make an outline on the card stock, and bondo on top of it. I used this and foam a lot on Infernis, there's about 2 inches between the actual skull and the case, it's filled with foam, cardboard from a 24 pack of Dr Pepper, and covered with Bondo.