When I put foot straps on my N20, I cut about 1" squares out of a 3" diameter PVC pipe. the radius of the PVC pieces roughly matched the inside of the hull. then I used washers inside of it. I also glued pieces of bicycle tube to the side of the PVC where it would butt up against the hull and used plenty of silicone. Never a drop of water in the hulls from the foot straps.