I had great problems water proofing this hatch.
The problem was I mounted the gas bottle hatch too high.
This stopped me from building up sufficient depth of ply to cut a descent drainage slot. This meant I had to drain the hatch forward (up hill). To overcome this, I fiber glassed in a piece of PVC tube and drilled some big hole down to it; which drains into the cockpit behind the hatch. I don’t know why I didn’t connect the drain to the bottom on the hatch; I must have had a brain drain!
Finally water ran across the lid, so I cut the drip slot in the lid. Problems solved!
Well until the first wave floods the cockpit.