Condutive clamp or similar?
Consider a foil connector (representative image below). These have integrated spikes meant to puncture a foil shield and provide a good electrical contact to it; I expect they'd work well enough on thin sheet as well.



Most are designed for permanent installation, crimping directly onto a wire, but if you want to easily connect and disconnect wires, there are some like this one that have a QC tab to mate with a common, cheap QC terminal that you can crimp onto a wire. Of course, you can also get one of the crimp-on ones and give it a pigtail with whatever connector you want, too.



For heavier duty, a split bolt might be an option. If you tap a hole in your sheet, or just use a lock washer, preferably the star type, (to penetrate the insulating oxide layer) and a nut with an unthreaded hole, you should be able to make good contact with a large-gauge wire.

Note that I haven't personally used a split bolt in such an application and don't know how effective it will be. I have only used them to ground chain-link fences, for which they work well, but I don't know how good a contact they would make through the threads, nor do I know how to determine what size nut to use.