So the pi zero presents a challenge with regard to turning it on/shutting it off. A direct kill switch on the 5v in is a great way to turn it on, but flipping it off leaves you at risk of corrupting your micro sd card. Having a pushbutton tied to a gpio that calls a shutdown script is great to shutdown the machine, but can't be used to turn it on. The 'run' pins can be used to reset the pi zero during operation, or, after it has been shutdown safely, used to bring the zero back to life.

The original case had a nice square slot at the front right, which is perfect placement for a switch of some kind. However, I didn't want to trash the original ethernet switch in case I decide to use it later in another project. So looking through the box of switches I came across a black momentary closed push button . Only problem was that it has a circular diameter of 12mm - not a rectacular 8mmx3mm slot.

Opening up a square hole to a round hole isn't too much of a problem with the right drills and some cutting fluid. I used a 10mm drill to expand the hole, expecting to further open it up with a 12mm drill. Only problem, I had no 12mm... So a combination of a number of flat and curved files produced the below result. Not too bad, and covered by the bezel built into the switch.