I wired up a rocker switch for LED lights that are wired directly to the power supply on my Ender 3 Pro, and needed a way to mount the switch. The box fits a rocker switch that is approximately 30mm x 20mm (https://www.amazon.com/gp/product/B07QR4ZKL8/). The screw holes line up with the v-slots on a 4040 extrusion, but are sized for M3 bolts. I used the M3 T-Nuts found here: https://www.thingiverse.com/thing:3050607.

The walls on the box are a bit thin, so may need to make sure you are well-calibrated, and you may want to use PETG instead of PLA, as PLA may be a bit too brittle at that thickness.