I have used foam, epoxy and fiberglass vacuum bagging for model aircraft wings in the past with great success. But when I tried it with a small play kayak (9'), I was unable to get decent strength with several layers of glass. I suppose that if you used the foam as a plug or mold you would be ok with multiple layers of glass. You might want to go under yahoo and search for "vacuum bagging" and read some of this material. It does produce some amazingly strong wings for planes, but then again, the wings are only a couple of feet long. :) If you get the problems worked out let me know I still want to finish my attempt.