I'm building a Guillemot and I came up with a flush hatch seal which is quite easy to build in comparison to creating a fiberglass lip. (30 minutes construction time) After cutting out the hatch, I created 3 rings of 1/4" ply, to create a recessed lip. the first two layers are 1" wide bands which have an I.D. the same as the cut-out so the hole is now 3/4" thick including the deck material. A 3rd ring which is 1 5/8 wide is the bottom layer. This ring stack is actually cut in two down the centerline so it can make the curve of the deck and epoxyed to the underside of the opening using 5 minute epoxy. I used spring clamps to hold it and in 15 minutes I was done. Now seal the wood with epoxy or whatever and install an automotive window seal which slides on the 5/8 lip and totally covers all trace of the 1/4 wood. I had some 94 Mercury Villager rear gate window seal laying around which worked perfect for the above lamination thickness. (the above instructions would have to be varied for another seal design, check the Mercury dealer to see what I mean) The bottom sealing area of the hatch door should be made smooth so the rubber can seal. The finished product is a factory looking watertight seal. Use silicone to seal the ends of the seal.