I've got a sheet metal nibbler attachment for a drill motor. A guy could set up a jig to guide it in a perfect circle arc. That might do the job for you. It cost me all of $25 and is two ended, so when the first side wears out, just swap to the other end and keep on going. You'd generate quite a pile of little sheet metal crescents with it for sure. Or a jig saw with a similar guide would do the trick.
This one comes with accessories for a bit more money.
Amazon link
