I needed one approx. 90 degree bend in a length of 12mm stainless that I will use as a rotating racking arm. I've never been shy about buying tools, but for just one bend I decided to try build rather than buy. After a little research I built the jig shown in the photos.
I took a scrap piece of hardwood, cut the radius with a bandsaw, then formed the groove with a hand file. The entire process took about 20 min. If you look at a commercial bender this is a hardware equivalent of the mandrel. The groove needs to be a snug fit the same size as the tube to keep the pipe from crimping during the bend. Common advice is to fill the tube with sand to help it keeps its shape. I didn't have any fine sand on hand and was somewhat concerned about pitting the inside of the tube. Instead I packed the tube with semolina pasta flour. This may not have been necessary. I just wedged the jig in the vise, no clamping pressure, then did the bend by hand. I couldn't be happier with the results. There is a slight tapering around the bend, but no wrinkles and no flattening.