I used this to mount a single channel 5 V relay switch to the back of my MK3 to control an LED light bar. The control is wired to a GPIO pin on a raspberry pi that is mounted below the einsy board and the 5 V +/- power is wired directly to the einsy board (on the pins that would power a Pi zero if one were attached). A 35 mm M3 bolt will sit flush with both the lid and the printer. There is a cutout to route the ground wire from the LED to the einsy board (the other is cut and wired into the NC terminals).

I printed it in PETG with supports for the hangovers (included in the gcode). The tolerance of the legs on the lid are very tight, but you can just snap them off since the relay board sits very snug in the case. You can screw in the M3 bolt to pop the support out of the bolt hole in the lid.