I had a large stainless steel holding tank
made for my Sun Odyssey 51 as it did not have any when i purchased her. She is a 1991 from the Caribbean so holding tanks
are not required in most ports there? But they are in CA
The tank could also be for fuel
with little modification. I have not checked the capacity but it is large. I designed it to go under my forward cabin bed area. There is a molded fiberglass triangular well area perfect for a tank. I have pictures for of the area and the tank for anyone else interested in doing the same thing. The dimentions are: 60" long by 35" wide at the widest part of tank (aft) and 14" deep with an additional 5" tapering to 3" trough/gathering well in the bottom aft of tank( where the pickup tube is place)
maybe Beneteau 50''s have the same area in the forward cabin sole. My email for questions is Myblueheaven@att.net The guy who made mine can make more easily. It took just a week for mine and it is very strong.