[Milsurplus] "N" bulkhead punch?

[W2HX]

Hi friends,

I am building an outdoor antenna control enclosure and I've decided to use "N" bulkhead connectors to bring RF in and out of the enclosure (no need to debate the benefits/disadvantages of "N" vs "SO-239" connectors)

Most of these bulkhead connectors have a "flat" side. This flat side prevents the otherwise round connector from twisting when tightening on a cable.  I don't have any special tools to make this type of a hole in my enclosure. I am guessing that a metal punch (like a greelee) should be used to make such a hole.  Does anyone know what kind/model of punch I should look out for? What size?

Any info appreciated.

73 Eugene W2HX