We have quite a few, not sure what they are called, metal things with triangles, which go into concrete bases, for stability, cut into length of about 1mt, they go rusty and make great plant supports. On the picket fence by our veg plot, I have got old kitchen sink tidies,(plastic )growing herbs. We have a round sink, there used to be a stainless steel "basket" in there, I put chains on it, and that is where my (indoor) orchids go in the summer, hanging from a tree.
Bunny Guinness (landscape designer) talks about using builders' bags to make planters. She advises to make sure the bag is even while filling (the example in the picture is a bit wonky). You can add surrounds like this heather or hessian. The bags drain well. You could use them to half depth or even double height, I guess.
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