The suggestion to use 2-1/4" or 3-1/8" hole saws is absolutely correct.
Those were the original standard instrument sizes (and you will find that
the P.C.D. of the fixing holes is also a neat measurement in inches.

Over here in metricland where they don't understand inches, it was
converted to the nearest integer millimetre.

And a hole saw (used on a panel clamped down to a good rigid drill press
bed for accuracy) is a damn sight less costly than a hole punch.

Although Bob's idea of a piece of det cord looks interesting, I must try
it. On someone else's panel...............