You'll need to go around 3 sides of the frame with some substantial thick walled tube to make it stiff enough. This is only if you need the metal to change shape quickly. The machine will work right out of the box, you'll just have to keep at each panel longer.
Besides the soft frame, the wheels have been known to be out of round. If they are too far out, they will only put pressure on the sheet for about 1/4- 1/3 rotation of the wheel and this will give you hard time to get the panel stretching at an even rate.
I haven't tried a wheel being out of round before and I sure wouldn't want to.