-- Last Updated: Jan-07-13 7:31 AM EST --
From the look and the vintage of the design my guess is that the larger opening never had a Tupperware type cover but a solid plastic cover in which there have been already a few suggestions. This looks like a cover that a friends Seacret used to have. It was a combination neoprene gasket/cover and hard hatch cover. Being that there's compression straps try a trimmed block of 2" mini cell foam. That should give enough of a seal for light duty use. As to the aft screw in hatch, I doubt that WS made their own. The topkayaker site will probably have a screw in cover that'll fit that rim.
Kudos to you for thinking ahead to safety mindlessness in introducing the next gen of paddlers.
See you on the water,
The River Connection, Inc.
Hyde Park, NY