Standard metal work?

the two easiest options are a punch or getting it professionally done. you can goto places like and have it shipped to them and done for a pretty small fee (a lot less than buying a punch!).

the only other way would be to find something that has an IEC socket in it already and cut out a square and bolt that onto your chassis.

OR, you could toss a nuetrik connector on there or something and have a custom terminated power cord that would connect to that round connector.

but after reading what you want again, finding a shop that could cut it for you would be your best bet.