Aft watertight compartment
After cleaning the tank, I washed it with soap and bleech several times, then wiped it with acetone, again several times, and finally I sanded it. Then I tabbed around the lip of the old tank that I did not grind off the hull because I was, at that time, nervous about grinding that close to the hull.
I build a cardboard template and made sure I was good with the measurement. I sloped the tank down from rear to front creating a sump about 10" deep at the front end. This is where my bilge pumb pick-up will be placed. Then I attached hardwood cleats with epoxy. I cut the top and front from 1/2" marine grade Douglas Fir plywood. I made a cut for a 4" inspection plate. I built up the opening 1" high with two "rings" of 1/2" ply I laminated together, then to the top, and tied them in with fiberglass tape. I covered both sides of the plywood with a single layer of 17 oz biaxial. I covered the top cleats with thickened epoxy and pressed the plywood down on the cleats. Then I tabbed the top and front to the hull.
I think it looks good. I have room to install a gray water tank on top of this compartment well below the cabin sole . . . or maybe a wine bottle storage locker.