This is an emergency shutdown and smoke detector automatic shutdown system. When smoke is detected the printer will automatically have power cut off to it. The red button will do the same thing as well as for a trip test button.
Update: I realized the lid needed more screw holes so I added a new version. I also added just the tabs incase you already printed it and they can be glued on.
I used Kidde kn-cosm-ibca smoke and carbon monoxide alarm with battery backup. The relay is Kidde sm120x. If you can't find that manufacturer or model I am sure most other manufacturers make a similar relay module compatible with their alarm models.
This setup has been in use for several prints now and has worked well without any issues. It has been reassuring to know that it is there if something happens.
For me setup I continue to use the normal on off switch that I had mounted on the power supply. It could easily be mounted to this unit also or you could put an on off switch instead of the trip test switch.