If you have a graphics program, you can make a template for the cutout, and then use that to trace the pattern on your box.
I start with a large drill, and then use a combination of a metal nibbler and a files to make the IEC cutout.
I think most IEC's cover up the hole itself, so they are forgiving of non perfect cutouts, you really can't see how bad it is

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The other thing you can do is forget the IEC, and just wire directly from the plug to whatever you are powering. If you go this route, you just need to drill a hole, and put a grommet in it so the metal does cut the cable. I would also tie it down inside the chassis some where to provide strain relief.
Randy