I made this to use on a few motion sensor lights, where people kept accidentally toggling the switch. It's worked great, and has enough resistance that you can't accidentally toggle the switch when the lock lever is in place.

Use the existing screws in your switch plate to screw this in to whichever orientation you want locked. You can push the lock lever out of the way to toggle the switch into the unlocked position and friction will just keep it in place until you toggle it back.

For durability you'll want to print this with at least three perimeters in PETG, ABS, or a tough (modified) PLA.