Just an additional consideration.

If you cut a large hole in a stressed plywood panel it changes shape due to the released tension.

Point one: keeping the cutout close to the bulkhead will reduce this effect, at least at one end of the hole.

Point two: If you decide to use strips to create your hatch-covers then you will get a more accurate curve if you laminate them on the deck over the cutouts after you cut out the holes. I would even glue in the hatch support rings first as this will change the deck shape somewhat too. Small differences they may be, but if you want to use thin gaskets you want accurate curves.

Etienne