John,
the manual for the Shearwater has you place the spacer for the hatch (flush mounted) on the deck to trace the outline to be cut out. This lined up on the deck side, but when I tried to fit the spacer, it wasn't working. I've since shaved down quite a bit and it looks like it will work in the end. I'll just make sure to use plenty of epoxy to make it solid.

Thanks for your response,
Gero