A 3'x7'x4'-tall box would hold 700 gallons, with an 11'x15' folded liner;
3 walls might be reinforced shed walls. The sideways pressure at the bottom
would be about 250 psf. You might screw a flat perimeter 2x4 to the plywood
bottom and sides with and 4 bolted horizontal double 2x4 hoops above that (1
flat 2x4 on the short walls, sandwiched between 2 on the long walls), with
some spiked mending plates to spread the bolt load and 3 single 2x4 hoops
above that, and a vertical 2x4 around the top. I wouldn't go higher than 4'.

That version would use 4 sheets of plywood and 35 8' 2x4s, about $143 with
the double poly film liner. Welded wire fence on the bottom with a 2x4 hoop
inside the box at the top and bottom and 26 4' vertical studs on 1' centers
would lower the materials cost to $102 and reduce the labor required.

A wall might look like this, viewed in a fixed font, with a few wires across
the top to hold it together:

---
| |screw eye + wire--- - - -
| | film
| |pp - p
| |ol|2|o |
|2x4|ly|x|l
| |yw|4|y
| | ofilm |
| | op
| | do
| | pl |
| | ly
| | yf
| |
... 4'

| | wi
| | ol
| | om |
| | dp
| | po
| | llyfi |
|2x4| y - l
| | w|2|m
| |wo|x|p |
| |io 4|o
| |rd - lyfilm...
--- efencewirefence... - - -

A tank like this could be much larger, with little increase in materials cost.

Nick