Hardening foam is expensive but... really hard.
Just google it. I /think/ FlexIt is a brand, and Fudz073 mentioned Great Stuff. If you check out Ben Streeper's thread, he has a few videos on it.
Or you could always just Rondo the whole thing. It's easier, and almost as hard.
But you can't really go wrong with fiberglass... I mean, it's what they make boats out of. If you do a thorough job, it won't give you any problems.