I had some difficulty getting an airtight seal on the aft hatch on my GRP Mk II Kez with the rubber in its conventional place glued to the deck.
With a bit of lateral thinking I have achieved a good seal putting a 9mm thick neoprene foam strip on the inside (underside) of the hatch cover, having made sure that the edge of the lip on the deck was flat (using two lengths of thread on the diagonals and a good straight edge looking at the sides). To flatten the athwartships camber (not often you can use that word!) of the hatch cover I obtained some closed cell polystyrene foam as used by architectural model makers and stuck 4 x 25mm wide strips along each side above where it rests on the deck flange/lip using epoxy with glass micro-balloons to make a gloopy paste in the underside the hatch cover and made a wooden jig to allow me to sand the surface of the foam flat and then sealed it with glass cloth and epoxy with more micro-balloons. The self-adhesive neoprene foam was then glued on with the corners very carefully mitred.
The result is that the seal is well protected from snagging and ultra-violet. I held 150mm water gauge for about 5 minutes before I got bored sticking my tongue in the tube to maintain the seal. Fortunately there was no one around to photograph me during the test…
Less success was had at the bows chamber where a “water-tight” hatch by a well known manufacturer failed to seal (smoke streaming past the O-ring) Only copious amounts of grease and serious grunting it tight shut have succeeded in sealing the hatch. I hope I don’t need to open it anytime soon.