Ronco Plastics
Ronco Plastics marine Tanks makes TOP quality thick-walled (50% thicker than most other mfrs) 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--including diptubes--in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.
When looking at drawings it's important to know that there is no top or bottom until the fittings go in, and YOU decide where they go. So rotate, flip in any orientation to find the tank that fit your space.
In the unlikely event Ronco doesn't have the size and shape you need--that you actually do need a custom tank, Triple M Plastics in Maine
Triple M Plastic Products Inc does excellent work. I know of many boat owners who've bought custom tanks from them, 100% of whom have had only good things to say. Send 'em a drawing for a quote.
--Peggie
