I cut the lips out of birch 1/4 inch plywood and clamp and epoxy them from beneath the opening. My lips are about 1/2" into the opening. The hatch covers get 4 layers of 6 oz. cloth on the bottom for stiffness. Fill the weave on the outsides to form a smooth surface where they will press against the weather strip gaskets.You could add a layer of thin 1/8 plywood spacer over the lips (lowering them) to insure that the hatch will be flush after tightning down. If the gasket is not too wide, they will compress level to the deck without the spacer piece.

I use the household weather stripping (closed cell) gasket with the adhesive back.

1/4-20 stainless threaded rod runs from a knob into a wing nut that is epoxied into a groove under a piece of 1/2" maple dowl stock. A small block of closed cell foam prevents the dowl from turning. That can be fastened to the deck or under the hatch cover itself.
   You slide the covers on forward and back, placing the cross beam under the deck. By tightening the knob (threaded rod) the cover gets compressed down on the lip and gasket. They work great!.

Other tips: Yoga mats (available in department stores) makes good gasket material. Fasten it with contact cement but do it wet so you can push it around a bit into position. A small knob is all that's needed. Rather than a wing nut, you could use a stainless T-nut if you can find them (James Town Distributors in RI).
Any closed cell foam will make a good gasket under the knob.To pull up flush hatches, you could have a pull cord or... put a nut on the threaded rod about a half inch below the hatch cover so you can pull up on the knob to pop them off. I buy 3" SS 1/4-20 bolts at the marine store and cut off the heads for the threaded rods.

 

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Flush Hatches
These hatch covers are easy to make, and are 100% water tight and sit flush to the hull lines with no straps or cords. This design uses direct downward pressure, insuring a good water tight seal.

I start my cuts by scribing through the wood with a utility knife against a metal ruler. I then finish the cutting with a power jig saw. Cut carefully on the line ...there’s no second chances.