> I had considered making small cuts into the underside of the plywood hatch so
> that it would flex more easily. Be careful doing that or you will creat grooves for the water to get into your hatch.

I built my covers by actually marking the ribs from the cutout of the actual hole, marked a line down the center of the hatch, marked another line down the center of the rib, slathered it with epoxy/cabosil and used c-clamps and wood clamps to hold it in place and they match the deck pretty closely.