I thought the design of these looked pretty solid with the extra cross support and easy to hit screw holes. I resized them a bit to fit what seems like US standards and rounded some corners.

Use these spacers to shim out your switch or outlet so the cover plate fits correctly. You might have to use these if you have added a layer to your wall like tile or if you have used the wrong size box. Adjust the Z in your slicer to your needed thickness.

I printed in PLA .35 layer height 20% infill

The single box OD is: 2.25 x 3.875 inches
The double box OD is: 4.125 x 3.875 inches