if not then..
I don't know the answer to your question but it seems to me the foam bumpers are made out of close cell foam. Exactly the same stuff they sell to make seats out of for custom made kayaks. Just go to your local outfitter store, like REI or EMS (if you're in the states) and ask them to cut you a piece to suit your needs. As I recall it's pretty cheap stuff sold by the square foot. Then cut it to shape yourself. If you've got an electric turkey cutter handy and can do it without your wife or mother finding out, that stuff cuts like buttah!
Good luck!