I did something similar on my Guillemot, and retrofitted on my CLC Yare, only it comes out flush with the deck. 3 years' worth of kid paddling and wet reentry practice up the rear deck popped the seams on both sides, so last summer I redecked it. Originally it was built right out of the book, with 3mm and no glass. The new deck is 4mm, with glass inside and out. After sealing the underside with epoxy, I glassed it with 4 oz. cloth and prestressed it into a half-round shape with duct tape running from side to side at 3 locations, with the curvature just a bit more severe than that of the deck. The tape messed up the glass in those spots, but they were cut off later anyway. I expected it to relax some after cure, and it came out about right. After installing the deck on the boat I glassed the complete top of the deck and over the sheer about 4".

Then I drew out the oval hatch shape on CAD (TurboCad 3.0, $50 off the shelf at Future Shop), printed it out, taped the sheets together for a template, sprayed the back with 3M 77 adhesive, and smoothed it in place on the deck. Cutting very carefully to the line, I zipped out the piece that would become the hatch cover with a saber saw. Then I made a spacer ring out of 4mm, 1" wide, same ID as the hole, and another with the same OD as the spacer and 2" wide, so it protrudes from under the deck 1" all the way around the hatch. This is all sealed with epoxy putty at the joints, and neat epoxy on the bare wood. Sanding ws done after the first epoxy coat so that I didn't expose too much inner ply on the tight radiused edge. Then I used some neoprene P-channel weatherstripping, the hollow kind that comes in 2 rows, from 3M. It zips apart, and one row went on the outer edge of the hatch cover, and another row on the inner edge of the inner lip. This forms a labyrinth seal, trapping water that leaks around the edge, and so far has been 100% watertight.

The preglassing and prestressing of the camber in the deck, the glassing in place on the outside of the deck and the long cure time have produced a cover that conforms to the deck camber without any braces at all. The 4mm spacer allows the weatherstripping to compress about 50% under strap tension, and does a darn good job of sealing. As an added bonus, we can use it for a plate when camping!

Hpoe this helps.