This is a small switch box that mounts to the leftmost 4040 extrusion on the Ender 3 Pro using the screws that hold the electronics box to the machine.

The switch I used is a KCD1-101 from DaierTek-https://www.amazon.com/gp/product/B07S2QJKTX/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 (the listing rates these to 125V, however I used it for a 24V LED I have mounted to the hot end so I can't verify the reliability of the rating).

I had to sand some of the edges on the backplate to get them to fit, so I added chamfers all the way around it to hopefully make installation easier.