All builders go through the same problems . Here's how we solved the for our boats. First our hatches are on strip biult boats , and the plan called for strip hatches made seperately. We did that glassed on both sides and eventuall the hatches started to show the glass through and leak. These hatches were made over seprate forms, more curved than the deck. to stop the flex problem we eventually went and mad them of 3mm ply over the same forms. This is how we staple or nailed a piece much larger than the decks to the form , nailing at the edge wich would be discarded. The we slathered on epoxy, let soak both pieces , put on some more epoxy mixed wwith sanding dust (wood flour) and put the second layer on top, stapling at the edges. when dry makes a stiff hatch cover cut out of the center. That works for plywood boats, or strippers with ply hatches. Next step, flush hatches cut from the decks of a stripper. We used the deck piece cut out with a very fine saw, cut oout fof the decks after they were glassed and on the hull. Made plywood rings and spacers for under the deck to get them fush. But agian to much spring. So we used the sme process, but after we cut them out of the deck, wewarmed them with a heat lamp, put slightly more cure in a strap, and added a second layer of glass to the bottom side. And it works! Al