Concerning custom water tanks:
I made a custom grey water tank for our Avion more than 20 years ago and it is still in good shape. It is nestled between the frame rails in the back just ahead of the factory black water tank.
I made it with 1/2 inch marine plywood, rather expensive but worth every penny in this applicaton. The bottom and three sides were reinforced with gussets made from pressure treated 2 x 4s miter ripped to 2 inches on a side. All was glued with waterproof glue and screwed with stainless screws.
A flange of 2x2 pressure treated was made around the top to screw the lid on. The inside was lined with thin black polyethylene, (I used one of the large extra strength garbage bag liners). The lining was wrapped over the top edges, then the tank was filled with water to stretch the plastic liner into the corners and crevices of the tank and stapled on the outside of the tank walls. A marine grade fitting was then installed for the drain.
A top was made with plastic covering the bottom side of the marine ply. Marine grade fittings were installed for the fill and vent lines and a marine access plate was let in for cleaning or servicing. The top was then screwed onto the tank using liberal amounts of liquid nails adhesive at the joint.
The liner sticking out from the joint was trimmed flush and the whole assembly was painted with a good quality marine paint
-- several coats. It is still just fine, fully functional and looks like in another 20 years would still be the same.
I have never had to clean, tighten, service or do any maintenance on this and would do it again. I don't see why it wouldn't work for any type of water tank. Boats use often use this type of construction for fuel tanks, except they fiberglass the outside instead of using a bladder on the inside. The fuel pickles the plywood so there is no deterioration.
Just thought I'd pass this on as a idea for getting wierd shaped water tanks at a reasonable cost.