I was using some 2" pink foam for the sponsons. It works pretty good but the next time I'm going to get some blue. BTW, I'm going to invest in a hot wire cutter so that I can just burn myself instead.
In our kit, we're using 1/8" balsa attached to the foam with double-sided tape on the sides then use a thin layer of epoxy to 1/32" lite ply skins all around.