I am not aware of any system specifically designed for this purpose.
The best I can think of is using a #10 bare copper wire, attached to the panels with the little spring clips that you sometimes see used on metal boxes; I hope they would grab the thinner metal.
For heavier metal, such as I-beams, I suppose a lug mounted to a beam clamp would work.
I say #10 for mechanical strength, and I'd run it inside, near the roof line for protection. I would not worry about a larger ampacity wire; #25 sheet metal can't carry all that many amps - any circuit larger than 30 amps is going to have to rely upon it's own ground wire.
