I may be wrong on this point, but looking at other benders that do square tube, if you wish to get a tight'ish bend on square tube I think you need a ridge on the middle bender to make the inside wall collapse in on it self. I think a ridge of 3 or 4 mm could be enough depending on what wall thickness you use.

If doing a large radius I do not think it would be an issue.
Also some guides to keep it straight., these could be rings you can move and clamp into position to suit the size of tube etc.

Just ideas to throw out there.

Adrian

I have just built a bender myself, of a slightly different design and desperately trying to make new formers for different profiles.