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Those kind of benders are intended for copper pipe through EMT (Electric Metallic Tube - the thin wall steel tubing for electrical runs). Thin stainless will probably bend OK in them. They won't last forever, but if you are making one for you (and maybe one for me ), they should fold up just fine.

You are right, precision tools they are not, but with care they would work and are a bunch cheaper than other options. Dies for my bender cost 5 to 10 times those complete tools.

I'd probably make ten of them once in the groove! But if the equipment cost this much, that want happen anytime soon
And I don't want anything of questionable quality for one time use.
I think eventually I will make a mock up from copper (I have copper pipe here), and also a bunch of copper and brass pieces to make the brackets and solder them together.
Once I have a model and happy with it, it will be easier to plan making a real one.