On my cape charles, the hatches have never sealed. I added plywood layers to the outside of the hull, built it up 1/4" around the perimeter of the hatch. The cover extends over that, with gasket. The straps didnt seal, and I went to shockcord inside. It looks nice, is a hassle to remove, and leaks worse. Trial and error, I said in another post. Mostly error in this case.

An internal hold down line would work well in the front, the bulkhead is right there and it would not interfere with storage. In the back, I'm about ready to find some stainless T-nuts and simply bolt the thing down. I'm fishing for better answers... Or a source of stainless T-nuts. Dzus fasteners wont work, the interior of the hatch has a beam there. Softer gasket will help, but I doubt it will solve the problem.