A nibbler is a good tool for case material. Dremel is fine too.
Most electricians 'round here call the punches Greenlee after the mfg.
Most industrial/commercial electricians would have them to punch conduit holes in service panels. ( you don't always have the right sized knockout in the right spot)
Some posts seem to be referring to router bits with bearing guides- exercise caution
as a router bit for ferrous metal is rare.
The air-powered nibbler is available from Harbor Freight (www.harborfreight.com) for $30 to $50. It uses a 3/16" nibbling bit, and you'll need a 9/16" hole to start. Just nibble close to the line you want, and Dremel or file the last little bit away.
You'll need hearing protection -- the nibbler is loud.