I made my coaming convex, i.e. it curves downward from the inside edge of the lip to the outside edge. I also made it so that it is concave underneath. That way the bungee for the spray skirt holds tight up under the lip, and it pulls the skirt across the surface of the lip making a seal. Also you may need to make your lip longer if it's not sealing good. The skirt should wrap all the way around and hold under the lip. It may be riding up on the edge of the lip... just guessing. I can roll over, stay there, miss a few rolls on the way back up, and not get any water in my cockpit.