The rocker switch cover is meant to be attached to a power strip by velcro wire straps or some other kind of velcro tape. For a more permanent attachment zip ties can be used as well.
With its inner dimensions of 20mm x 25mm x 10mm it fits on all power strips I have at hand. If you need a different size, use the customizer of OpenSCAD to make it fit.
There is a version with holes to let the indicator light shine through. If you are using a transparent / translucent filament, your power strip doesn't have such a light, or you want to cover it, use the regular version.
The blue one in the picture is printed in PLA, the other one in PETG.
"0.20mm Quality" preset of PrusaSlicer with modified extrusion width (Advanced):

19 min for either version with modified extrusion width or 22 min with unmodified 0.20mm Quality profile.
The author hasn't provided the model origin yet.