the two easiest options are a punch or getting it professionally done. you can goto places like www.frontpanelexpress.com 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.