: I also have some polyisocyanurate. It's stiffer than the
: polystyrene, but I'm not sure how well it'll bond with glue or epoxy.

Polyisocyanurate is rather brittle, so I suspect is would degrade rapidly in a flexible object like a paddle. It's also really nasty to work with. I once had a job where we were insulating temperature cycling chambers with it and crawling around in that stuff all day was not fun.