ballard sheet metal did up a 60 gallon stainless steel diesel tank for my sailboat.

They did nice work and were the most reasonable of all the places I checked with.

you might consider prefab plastic. they make alot of different shapes and are really easy to do yourself.

My dad and I did his tank about 15 years ago, and he hasn't had a problem since... It only took us a weekend including the fiberglass work... Its really not too difficult a project.