I've learned a neat trick from one of the videos on the internet: you can fill the square tube with sand before bending it - to minimize the squashing effect. The sand keeps the walls from collapsing.... you should probably heat the outside of the tube to allow it to stretch.
Secondly, if the esthetics don't have to be perfect, you could cut small notches on the inside of the bend - this allows the outside of the material to stretch normally and the inside to be compressed naturally into space (where some of the material has been removed) - once you achieve the radius you want, you can then weld over the cuts and smooth out with the grinder - that's what I did for my sliders and it worked fine - since I didn't have a tube bender.
'95 FZJ80 with Lockers; Home made sliders - and it shows...
285 DuraTracs; Seat covers, CB radio