I have a late 80's Morgan
43 (hopefully similar enough to give you an idea here) and the previous owner turned the aft locker behind the aft stateroom into a spare fuel tank
(still not quite what you're doing but close). They just glassed in the locker about halfway-ish down, ran the fuel
line from the bottom to the engine
room and that was it as far as I could tell. The marine surveyor
estimated they added about 35 gallons of fuel
I can tell you that this was a very bad idea or bad execution of it. I don't think they were really able to seal it properly and it got contaminated pretty badly. Badly enough they simply disconnected it - I think water
was in it constantly and it was a pretty nasty mess to clean up and restore it. We had some diesel
intrusion into the aft cabin
too. Not much, you could just catch a faint whiff of it if you were messing with the steering
I'm not sure I would go this route
based on what I saw of it but I think the previous owner did not properly seal it so you may be able to get better results. If yours is like mine, you'll need to hire a midget or a contortionist with a high tolerance for tight spaces to get in there and do the work with any degree of quality or attention to detail. I think I would cut into the locker from outside somewhere and then seal a "real" holding tank
made from polyethylene or something in there if I were to do something like this which would be pretty pricey.
I put a new holding tank for the aft head
under the aft berth. It's awfully small, something like 22 gallons, but that's all I could fit in there and even then we had to cut up the bed
a bit to get one that large in there but after reassembly it looks like nothing ever happened.