I made a fixture to bend the bows on my canopy. I don't have a picture, but it was simple and effective.
What I did was take two pieces of wood (probably short pieces of 2x4), and cut a large radius on the end of one. I left the other square. Then I screwed each one flat to my workbench. The configuration was something like this (the D is one block, the [] is the other):
[]
D
Then, I placed the portion of the bow which needed tweaking where the tube rested against the round side of the "D" block with the end against a square side of the other block. Essentially, the tube goes on the tangent between the two blocks. I grabbed the free end of the bow and tweaked it, using the round block as a bending mandrel with the flat block holding the far end.
A few tweaks (not big honkin' bends), then refit and do it again. Remember, tiny little bends. Get aggressive and you'll kink a tube.
With this setup, some tubes are easy to tweak. Others are a challenge, but a little imagination will help...
I also tried rachet straps but there is so much spring in the canopy I could never tell if I moved the frame.