Mix some bondo in a stryofoam cup. I think you will find that the two materials an incompatible. There was an earlier discussion of materials here on this site, but I don't have the immediate link. Do a search.
On grill blocks, you don't want to do anything to make them permanent. Most folks have to remove all or part of their grill blocks in hot weather.
Though the following link does not address materials directly, there were several posts which did have valuable information on materials:
http://ecomodder.com/forum/showthrea...ght-12153.html
Coroplast is a very nice "sheet" type material. It is the corregated plastic used for signs. If you can find some old political signs you might FINALLY put them to good use ;-) Also, cardboard can be used for short term experiments.