Keith wrote:

Did you check Ronco for a tank? They make more stock shapes than anyone I
know, and are very easy to work with. http://www.ronco-plastics.com/.

I'd stick with regular steel for the tanks if you can't find one at Ronco. I
have a pair that are 17 yrs. old and fine. You are right about the inside,
the diesel is a good anticorrosion treatment, as long as you keep the water
out. The outside should be painted with a good coating of some kind, and the
bottom should be supported so that air can circulate under it. Obviously,
you've got to keep the water off the top of the tank as well. Be sure to
have the maker put a sump with a dip tube from the top in the tank so you
can suck out any water that might end up in there.

In a sailboat, maximizing tank size is going to result in an irregular shape
with the fuel pick up at the deepest point. A sump is an unnecessary
embelishment. Water is going to be of concern only with contaminated
supply in a well designed system. If water in the fuel causes sleepless
nights tee into the fuel pump discharge so a liter or so can be pumped
into a container for disposal.