Thanks!Hope this helps, first image is the cover right side up. Then upside down . Then a sketch of the hatch upside down. It's a 1" thick piece of plywood, 35 x 30-3/8. Edges have grooves for a rubber seal. And there's most of a 18-3/8 diameter circle cut offset in the cover. Sorry for the messy handwriting, let me know if you can't read it.
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