Brilliant Gentlemen!
I didn't use marine grade plywood when redoing the floors, so I didn't want to go full size on the 4" hatch.
If I ever have soft wood I'll be able to upsize to 4" and later 6". By the time that goes out, the boat will be "swimmin with the fish".
Thanks for the idea, and also the warning!
which allowed a slow leak to empty gallons of water under my floors as evidenced by the sloshing I heard
Wasn't there foam under the deck to begin with?