I also recommend Ronco Plastics (not to be confused with the VegOmatic Ronco)
Ronco Plastics marine Tanks They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank.
--Peggie