I remember reading some posts from someone who used silicone sealant ONLY. I think the process was to cover the hatch edges with wax, or some other release material, so the silicone didn't stick to the hatch, then smear silicone on the retainer lip, and press the hatch onto it, leaving it weighted until the silicone cures. This apparently produces a very thin seal that matches the contours of the hatch exactly, with no other gasket required. Might be worth a try. An archive search might turn up more details.

Ken