When I made the big hole in my popcorn can (my "mailbox mod") I used my saber saw with a metal blade. I backed the metal with wood to keep the thin metal from chattering.
Other tools I have used for this sort of thing:
Tin snips. These come in all sizes, and you want a small one.
Dremel cutoff tool. I love these. They'll cut pretty much anything.
Nibbling tool. This was one of the first tools I ever bought because I was making electronic projects, and all the project boxes, back in the day, were sheet metal, not plastic like they are today. It is a specialty tool, but it "works a treat," as the English say.
Keyhole saw. Depending on the model, some of these come with metal (small teeth) blades.
[edit]I looked up Fein Tool, and now that I see what it is, that would be a great tool for the job. I borrowed one of these for cutting sheetrock and it was absolutely amazing. There are a variety of heads for different jobs. They aren't much money, and I keep telling myself I should get one, although the guy I borrowed it from owes me a lot of favors, so I may just keep mooching from him.