I use the biggest drill bit I have to start, and get as much out as I can this way.
After that, I use a nibbler to take out the rest. I usually am not patient enough, and take out a little too much, so you can see it.
But, IEC's are forgiving, they have a plastic edge that hides most of your mistakes.
You can make a pretty bad looking cutout, but it will look fine with the IEC installed.
Randy