I know it's far too late now, but if you paint your foam with latex house paint, it'll stop the resin from attaching itself to your plug. HTH. Another thing to try is Duratec Surfacing Primer. It'll build up to 1mm (actually higher, but that's the max 'recommended'), and will sand to a very high gloss. Resin won't attack it, you can polish it, wax it, and then take a perfect mold from it.