Cutting and welding polysty
A lot has changed (I’ve learned a lot since), but here is a video about cutting and welding polysty to make containers of any arbitrary size, from, say, a cubic foot to 4'x4'x8' containers.
But whereas you can of course make a ‘large’ container— anything larger than about 15" to 18" deep— using only welded polysty, the fact is that the weight and pressure of water at those depths and deeper will break a polysty-only container apart. If you fill it with water, then as the water gets deeper, increasingly, the container will bulge, trying to become round, and that will put a lot of stress on the corners, which would prefer to remain square. Eventually the stress will break the corners apart from the top down. Hey, it’s simple: try it yourself.
You will find that what you have to do is take a step that is not shown here: You’ve got to support the polysty on the outside with wood. And because it will be hard to keep the lonely outside wood close to the polysty (which will still want to bulge), you need to ‘sandwich’ the polysty between wood on the outside and the wood on the inside, then screw outside wood to inside wood, strongly: a “polysty sandwich”, AKA “poly-panel”.