[Bernica]

I'm with the guys above about the RotoZip.
There is another tool out there that is used for cutting large holes in sheetmetal. I have one that I used when I installed Air Conditioning in our house, and occasionally since. Works on the same principal as the RotoZip, and uses rotary-cutting bits, but it's heavy duty and set up only for round holes. It's called a Malco HC-1 Hole Cutter. Mine will cut from 2" to 12".