I sometimes use my basement as a darkroom and need a way of indicating to my housemates that they shouldn't try to turn on the light; this is that.

This is printable as two parts:

1. A plate you can screw to your wall switch.
2. A clasp (attached to the above plate by a hinge -- use a ~1.5mm pin for connecting the clasp to the plate) that, when closed, prevents the switch it is attached to from being switched from its current position.