Hand bending 1" square 18 gauge steel tubing to a 4-1/2" radius
The key to this jig is a piece of 1/8" square wire, which is roughly centered on the outside of the jig. The wire could actually be round and anywhere from 1/8" to 3/16" for this job. The idea is to push the inside wall of the tubing in, and once it's going the right direction, it will take care of itself. Without the wire or something even more elaborate, you won't be able to bend this material tighter than about 15" radius without kinking.
If you look closely, you might notice that the straight part of the tube that I'm bending from ends up slightly curved also. If this is a problem, you can overbend the material slightly, and then carefully unbend the side that you used as a lever. But if it has to be perfect, then go buy a big expensive machine.
Oh, and doing this sort of thing is why I now have arthritis. And on that topic, the suggested youtube keywords to add are "rage against your hands".