Invent one. Use some scrap 1/4" plywood, size either the outside or inside to properly fit (once you''ve built out the glass) the hatch, paint the plywood with resin, mold release the painted areas (after they''ve dried sufficiently), take some 1.5 oz mat and start the process. Make the exterior final layups (inside and out) with the mat so they''ll sand smooth, and try 1.5 oz with 22 oz biax, then 1.5 oz, then 22 oz. biax and finish with 1.5 oz. Sand smooth, fill with filler (edges), and paint. Leave the center area raw so light will go through to belowdecks. Two layers of biax and three layers of chop ought to give you something you can stand on. See "This Old Boat" for further instruction.