ive got one of those cubes, have not had to use it yet. but what you are wanting to do sounds totally doable, box it in with wood structure would support the sides from bowing out.
I have one where the cage might as well be off... It's missing 1/3 of the horizontal bars and all but one of the vertical ones are bent, broken or not in contact with the tank. I have filled it up and it seems to be fine. Someone told me that the cage is just for transportation. If that's actually true, I don't know.
A friend gave me one without the cage. I filled it 2/3 full of rainwater and put fish in it and it promptly bulged out and cracked horizontally mid way. So I built a wooden frame around it for support and put a plastic liner inside and still use it but it was damaged quite a lot. Guess they need that cage for support if holding liquid.
With appropriate microbes, minerals and organic matter, there is no need for pesticides or herbicides.
Yeah, I had the same thought. But to me they're way more valuable as mega compost bins and storage tanks that I can move around. I also don't think I could sell an industrial square pond to the keeper of aesthetic standards, who is also my better half.