actually used the cheap "home depot" pink foam with Epoxy resin and several layers of cloth. Polyester resin wont work with the cheap foam as it will melt it into goo... First thing I did was create cardboard cutouts of the shape of the bulkheads by inserting in the hulls at the desired locations and trimming to fit. Then trace the cardboard onto the foam and cut the bulkhead shape. Layed up 2 layers of cloth and resin on wax paper, then the foam cutout over that, then 2 more layers of cloth and resin. One last layer of wax paper and some weight ontop to apply pressure when curing. Came out sweet, made 8 bulkheads (4 per hull) that way. 2 bulkheads for each of the forward hulls, 2 for each aft hull, immediately in front and behind the dagger wells.