Could the water be coming in from someplace else than door? Does it only appear to get wet when raining? Have you treid covering the hatch with plastic sheet and good tape to stop door infiltration and see if it get wet inside. If it is a door leak issue there are rian sills you can buy form over doors and hatches.
Still don't understand what the two slots are really for? Is it so you can remove the tank? There are totally plastic hatches with good seals out there made for the marine industry maybe it would be cost effective to get one of those and put you gas escape hole thru the bottom.
ExamplesGood one
