Foam might work
Posted by: rjd9999 on Jan-03-13 12:03 PM (EST)
It's kind of an odd idea, and I have no idea how well it would work, but one might try making a hatch out of sprayable insulated foam for the oddly shaped hatch. A plastic liner (trash bag molded to the shape of the hatch, for example) could be used to cover the hatch and the foam sprayed over it to make a mold that would be a decent waterproof fit.
Great Products from the Buyers' Guide: