It's really simple. My flush hatches sit on a lip. The lip is 1/4" plywood which I epoxy under the deck with squeeze clamps. It protrudes out about 3/8" all around. It bends into the hull shape. The hatch cover fits into this. I suppose you could do it also with the Chris Culzicki type of hatch that overlaps the deck

I drill a hole in the center of the hatch cover. I run 1/4 x 20 threaded rod through the hole that goes into a piece of 1/2" maple doll stock thats about 1/2 inch longer than the opening on each end. A wing nut is epoxyed upsidedown on the bottom of the doll stock. I put a block of close cell foam on the back of the opening to prevent the doll rod from turning. The doll rod positions itself in a bow to stern direction.

I use a stainless bolt which I cut off the thread section (stainless threaded rod is hard to find.)On the end of the threaded rod, I epoxy a knob. I use a maple toy wheel that woodworkers shops sell for toys. Once the rig is loose, You just slide the hatch cover and rod assembly forward, and lift it out ...and you put it on the same way ( sliding it on and then positioning it and tightning the knob. The doll stock won't turn since it hits the foam block just to the side of the back.

I cut three pieces of inner tube rubber to the size of a quarter and puncture a hole in the center. I stretch these on the threaded rod below the knob and above the deck to seal the entire thing. First time I used one of these, air was hissing out since pressure developed from the heat. Nick witnessed this in amaizement. I now drill a tiny breather hole in my bulkheads. Seal all holes with epoxy by first drilling a larger hole, filling it, then drilling a small hole in the epoxy. If I every learn how to sent a drawing, I would send one. Good luck.