Today I bent all the ribs. It worked quite well. I soaked them for a while in cold water, then bent them on a propane-torch heated iron tube. The previous times I had occasional problems with breakage. This time I pre-warmed the ribs, which helped avoid any breakage. I also managed to avoid any burn marks.
I have occasionally looked at the electrically-heated rib benders, but I doubt I'll ever get one. My current setup works well enough, and at the rate I'm going, a standard propane cylinder should last me for at least 10 years!