I made rounded triangles out of 4mm ply, used 6 corresponding to three hatch straps. I think I used ss flathead screws through ss. finish washers a piece of 6mil plastic as a washer (thicker would be better)between dog and deck through the recess lip and spacers to washer and locknut. Holes through deck were well sealed and screws were sent through with silicone sealant. It sealed as well as any other just with weather stripping gasket material, but used rubber U shaped gasket on recess to further crank it on. Turns out that put too much pressure on the edges of the hatch so it's a little cracked there. It might be a good idea to spend a little effort to reinforce the edge of the hatch with some extra glass. The screw went through one lobe of the triangle, the idea being that the dog had to be turned nearly all the way around to become loose, I suspect a circle with a semi-circle taken out would be ideal for ensuring that accidental bumping wouldn't knock it out of position. The one I made on a flat deck is absolutely water proof, I don't know about angled ones, you'll want to make the gap fairly even.