fuel cell?

You could make your own, does not need to be double wall.
Your size 8x8x8 really throws me, what would that hold 2 quarts?
In the tank you need an approved fill piece and a vent port on the top and a supply and return port somewhere near the bottom. You can purchase all these pieces and foam to put in it at summit.
You also can purchase an aluminum cell with every thing included it it for less than 200 bucks, but you would'nt have the satisfaction of making your own.