I have actually tried to use aftermarket foam like you suggested in the first post. I had a soft spot on the inside of a 16 hull that was about 3 feet long, discovered it one day after we ventured into 45 mph winds (go figure). Anyways I cleared out the foam and shot this stuff called Great Stuff into it. It worked pretty well. I then sanded down the excess, and put a couple layers of glass over it. I must say that now it's hard as a rock and although we dont' sail that boat anymore we never had any problems with it.