This is an FDM s̶l̶a̶ friendly remix of someone elses design. It should fit standard North American light switch covers. Prints in 2 pieces with supports as needed for the cover.
Update: Brain slip, i originally posted this as SLA Friendly, It probably is, but its designed to be FDM Friendly, not SLA.

General Safety Warning - You are responsible for your own actions. If you don't know what you are doing then don't try it.

To assemble:
1: Remove the screws from the existing cover plate and place the printed base over top, re-using the screws to attach it to the switch cover plate.

2: Hold the cover with the cross bar toward the loops on the base ~45degrees open.
You should see the flats on the cross bar align with the slots in the loops.

3: Moving the cover plate at a slight angle, slide one side of the cross bar into the matching loop of the base plate. Then align the flat on the bar with the slot in the other loop and firmly push. Be careful, especially if you have poor layer adhesion as the loop may break and require a reprint.

4: The cover should now be captive between the rings, but swing-able on the hinge. The design has a slight latching action when the cover is lifted and it should hold itself open if you wish. This wasn't an intentional feature but its a happy accident.