I helped a friend redo his hatches, tank covers, etc. on a 24 Grady several years ago. We just cut 1/2" plywood the size of the hatch but small enough to not interfere with the hatches closing. Glued the plywood to the underside of the covers with thickened resin, then just painted the plywood with several coats of resin for waterproofing. Holding good so far.