I remember a post (perhaps here?) where the author used some reasonably stiff wire to get the proper shape for the interior bulkheads (i.e., (a) put the wire against the hull at the desired location and form/bend it to match the shape (b) carefully remove the wire taking care not to distort and (c) use the wire as a guide to mark out the proper shape for the bulkhead). It seems like you could use this technique to duplicate the proper curve under the deck and then transfer the curve to the top of the hatch reinforcements.