But, if you are using an overlay as shown in the previous post, you don't need a keyway in the panel hole.
Just drill the extra locating hole for the washer tab into the panel and mount the washer on the rear side of the panel. The overlay will cover this extra offset hole.
I just included the male key in my switch holes when I designed the panel. Came out fine with the waterjet and no washer was required. I left keys on both sides until I placed all my switches in case I wanted to swap positions. I just filed down the key that was not needed.