In a non-thinking moment, I had an accident with the mill because the head had not been clamped fully. I found myself fumbling for the power switch and the incident reinforced the need to fit an emergency stop button. I had procured parts for this a long time ago, it was time to put them to use!
Above is an inexpensive emergency stop button from eBay, about $6 including the box. This switch had NC and NO contact sets, for this application only the NC set is used. A gland is used in the bottom of the box to let a 3 core 1mm^2 flex into the box.
A 25mm hole was drilled in the rear of the box, and a couple of 5mm holes to mount a surface mount socket ($3).
A couple of 6mm holes drilled into the protected area in the side of the box was used to mount the box to the mill stand. No accessible conductive fasteners were added to the box interior, so so insulation issues arise.
The total cost comes to about $12.