The main advantage of Closed cell is that you can cut a sloppy job and force fit it into a kayak with some glue. Closed cell is expensive, heavy, obviously hard to find etc. Since you're building not outfitting a plastic boat, try insulation rigid foam which is available everywhere. One inch thick stuff is very strong expecially if you coat it with epoxy and/or glass. Get the blue or green stuff not the polystrene expanded (popcorn) type. If ease of work is your main goal, you can still cut it - not on the money and force it in with silicone calk etc and coat it with a long brush with epoxy.

Many people pick closed cell because they automatically think it what you're supposed to use. Use anything you want.