You can get HDPE (High Density Polyethylene) tanks in all kinds of shapes and sizes...not just rectangles. I had the same situation as you a number of years ago where the seams on my stainless steel
tanks on a boat built in 1975 started leaking.
The stainless tanks were custom shaped tanks that fit very well maximizing how much water
they could store. I was surprised when I found some HDPE tanks that came relatively close to filling the same space as well as the stainless tanks did. I don't remember which supplier, since this was a number of years ago, but there is at least one company that can make custom shaped (welded) HDPE tanks, just in case you can't find anything that comes close enough. There are a lot
more shapes and sizes than what West Marine
has to offer. I found them by doing a Google
I would imagine rigid tanks are going to be more durable having a thicker wall combined with the fact that they will not chafe if secured properly.