Be careful with spray foam. I use Great Stuff spray foam on the bottom of my seat to conform to the haul shape. I coated it with epoxy and attached velcro to it. A couple of weeks later I lifted the seat out of the hull to check the foam and it had degraded out of shape from a reaction to the epoxy.

I fastened a couple of bouys in my bow and stern for my floation. Cheap. But probably not enough floatation for the average kayak. Mine is a special situation where I want my kayak to flood about halfway full and my side compartment floations act as pontoon and the boat becomes very safe and I can inter back into the boat from the side and not have to use a paddle float. My small bouy float are only there to give some stability at the ends. Just an idea.

John