A cover for a light switch with a latch to keep people from flipping the switch. You can also lock it by putting a small lock through the hole, or anything they'd have to break in order to open the guard. Obviously, this won't keep anyone from using the switch if they really want to, but you can at least make it so you'll know if they do.
I had trouble getting good prints with automatic supports, so I've included versions with the custom supports I created. They worked well for me, but as always YMMV.
This is intended to use a scrap piece of 1.75mm filament for the hinge. Heat up a piece of metal (I heated a putty knife with my heat gun) and press it to one end of the filament until it widens and flattens to your liking. Then put the piece through the holes and carefully flatten the other end, hopefully without melting the guard any in the process.