Thanks all, for your interest. Mike, The concave "rolling recess" is supported by a ply frame (#3 from the stern) defining the back end of the cockpit and a cross beam on top of it. To follow your advice, I will add another cross beam to support the back of the hatch, and two thin longitudinals to support the sides. I think that the main hatch, behind the cockpit, will be similar to one I saw last February on a "Long Shot" (by Jeff Horton) built by Peter Ingram-Jones in Hobart, Tasmania (website is Canoes and Lampshades) He builds SOF boats as light as he thinks he can, and could not consider using commercial hatches weighing nearly half as much as the kayak. He made a light frame for the hatch, skinned and sealed (with Corey's Goop I think) the boat, and then cut three sides of the hatch in the skin. The flexible skin is the hatch, and it is sealed by 1" wide Velcro, one strip glued to the underside of the skin hatch and the other to the frame under it. The skin is flexible enough for the fourth side to be the hinge. he says it is watertight enough for his use, but I might add D weatherstripping as well.
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Peter