Reconsider your decision about non-potable water. Going to such lengths for a tank and NOT having it potable, especially when you are in the desert southwest just doesn't seem very smart / efficient. It's a lot of volume and weight to carry, just to have to carry potable water too.
It wouldn't be pretty, but you could readily nest some large diameter PVC pipes in a honeycomb pattern to fill your desired tank space and plumb their ends together to create a single volume. Sched40 3"-5" dia PVC has a burst pressure of ~200psi. More than adequate to apply a moderate pressure to the tank for your shower.
Also consider aluminum tanks. You could have one made to fit your space for not a lot of money. There are also cylindrical tanks that may serve.
5gal 8"x24" is $130
Lots of options for cylinder tanks in the racing / offroad buggy world, which woul dfit in your space. And they are pretty affordable.