I've actually had very good luck trepanning sheet steel using a Malco circle cutter.

http://www.amazon.com/Malco-2-Inch-12-Inch-Cutter-Accessory/dp/B000289542

The Greenlee punches would be great, but very expensive.

I've used my Malco to cut up to 8 inch diameter circles using a hand drill. It worked surprisingly well.

Astrobuf