At a car and boat show in Franklin, MI there was a "Curley Craft" wood boat with the type of hatch I am building on my boat. Incidently, "Curley Craft" boats were produced at Northwestern Boat Co. on 10 Mile at Evergreen. The place is still there selling boats, outboards etc. and providing marine services, but they no longer build boats.
A three sided frame was built and added into the space to create the hatch surround. The rear beam was doubled up to provide enough surface for the plywood decking.
Then I traced the curvature of the rear beam and made a single hatch assembly that will later be cut into two hatches after the decking plywood is installed.
The hatch is mounted into position using some shims, duct tape on top to prevent drips of epoxy from gluing them in) to equally space it in the hole. The plan is that once the decking is in place, holes will be drilled up from the bottom at the corners to define the margins and then sawcut out the hatch. The side margins will be cut first and the hinges installed (while its perfectly aligned) and then the remainder cut out. Then the hinges can be removed and the hatch cut into two pieces and finished out. Stainless steel banding will cover all the margins.