Have you considered using a galvanized livestock water tank instead of building one?
I know if you build one you can make it any size you want but they come in different sizes, you could also use 2 or 3 of them.
This one holds 300gal>
Oval Galvanized Stock Tank, 3 ft. W x 8 ft. L x 2 ft. H, 300 gal. Capacity - 2177285 | Tractor Supply Company
Not sure if this is usable for what you want, just tossing around ideas.