I notice that no one is mentioning using expanding foam for the seat form itself.

I was thinking about putting on the wetsuit, getting a couple of garbage bags, filling them partially with the expanding foam and sitting in the kayak, (or have an assistant pour the foam into the bag while I'm already sitting in the kayak). This way I get butt and kayak contours at the same time. Thinking about making it in two halfs, and letting it expand up the side to form hip braces as well. This would make removal for cleanup easier as well. Eiether glass the foam once it sets or use as is, or perhaps a thin sheet of minicell foam for some cush, and to hide all the ripples left by the folds in the bag.

Am I missing something here? Sure there won't be much foam (if any) at the lowest point, but that helps keep the center of gravity low as well.

Yes the expanding foam is nasty, it releases fumes, and it will expand its way into and stick to any fabric or anything porous, and it releases heat while it cures, the wetsuit should provide enough insulation to avoid burns.

dave