A while back I cut out the holding tank that was built as an integral tank in the very aft end of the bilge. I am not a big fan of integral tanks though I think, under the right circumstances, they make sense, and are safe using the right epoxy. However, the holding tank was not epoxy, it stank, and it couldn't be repaired because it was inaccessible. I thought for a long time about what I would do with the space. I decided I would lower the top, make the space smaller, and turn it into a watertight compartment. This would provide some protection should we every get "holed" in the last 40" of the keel right up to the rudder shoe.
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 .