I can't believe I'm having trouble finding chassis punches for the IEC C13 socket. This has to be the most common way of getting 120VAC into a box, yet I can't find any chassis punches.
How do most people make a nice, clean hole for the C13?
I would make sure to check catalogues of Greenlee, Southwire, Klein, or whatever the major electrical distributers in your area are, as I've seen some oddly shaped and specialized knockout punches in the hands of data guys over the years. Greenlee definitely has rectangular KO tools. Failing that, you can certainly drill and file or use a rotary tool, but I would consider getting a sheet metal nibbler for anything up to 1/16" soft steel/iron. You can easily and accurately cut pretty much any square edged hole with one of these so they're great to have around.
I couldn't find a rectangular punch with that distinctive taper at the top, but I did find posts on a forum indicating a brand named Q-max may have sold them at one point. If you're operating in sufficient volume to make it worthwhile, you could likely have one made, or you could start ordering a version of the connector that fits a standard round or square hole.
If you're looking to acquire a decent hydraulic punch driver, now isn't a bad time. The recent advent of quick release battery powered KO tools has pushed a lot of perfectly functional hydraulic sets into the second hand market. Hope this helps.