I second Steve's recommendation
Ronco Plastics makes top quality water and waste tanks for a very reasonable price, and has more than 400 shapes and sizes, over 100 of which are non-rectangular...and they do install fittings in the sizes locations specified by the customer when they make the tank. The drawings for their tanks are on their website at http://www.ronco-plastics.comYou do have to pick the one of their drawings that best fits your space and go with that...you can't just call 'em and expect them to come up with it. And what you see on their website is what they have...they can't take "tank a" and make it 2' shorter and 3' wider.