Here are some details of the hatch setup I put in my new kayak. Its my own design, and man was there a lot of futzing while I figured things out.
This first shot shows the frame assembly, its basically two glue-lam rings that nest. I just cut out a couple ovals out of MDF and wrapped it with 2mm spruce strips. The inner ring has a few extra strips added to make a large flange on top, as well as a thick center stringer on the off chance that my split deck stringers fail and the hatch gets subjected to compression force. Dimensions are 8x10" for the opening. I tapered the inner ring so it would locate easier into the outer. The extra line of holes was from an early plan to fully wrap the hatch with fabric, which would have messed with the clearance.
Sewing the hatch, the first attempt I wrapped the fabric over the ring, and down the hole, it resulted in some serious clearance issues, so here I am just saddle stitching it to the side of the ring. Later it will be trimmed, rolled over, and whip stitched.
I covered the hatch with the same skin as the hull, you get some wrinkles on the underside of the flange, but it doesn't matter as they will be hidden by the gasket.
I trimmed the fabric on the hatch flush with the inner corner. You have to watch out for any large ripples as they may affect the clearance with the inner ring. I ended up using a soldering iron to melt down a few lumps that got in the way.
This is the finished hatch in its open state. The gasket material is EPDM rubber weatherstripping(the same stuff as car door seals). I put one strip on the hatch, and a second on the rim just for extra water tightness. I first tried closed cell foam weatherstripping, but found it took too much force to compress and didn't give a satisfactory seal, it also made the hatch look like an inverse Oreo. You can also see that I put on a latigo idiot strap so I wont lose my hatch at the beach.
And the hatch closed and secured. I used two latigo strap loops, one on each side, each has a slider, and one has a toggle. Its set up so that you can just barely loop the strap over the toggle. If you make it too loose the hatch wont seal properly. If you make it tight, you can pick the boat up by the hatch and it wont move at all. If I was doing it again I would make the toggle overhang a little less.