Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4944944.
This box is designed to hold one or two power switches for additional accessories adjacent to the Prusa Enclosure V2's mounting position for the black PSU that ships with the most recent i3 MK3S. In my case, I have added a separate switch to control the LEDs inside the enclosure, while powering them directly from the printer's PSU.
It could be mounted anywhere, not just the default PSU location - it attaches to the wood of the Lack table, not the PSU directly. Use the same size of wood screws you used elsewhere in the enclosure build.
In particular the holes in the front are sized to fit these easily available rocker switches, with a required hole size of 12mm x 18.5mm + tolerances: https://smile.amazon.com/dp/B087PYW9BS
If you only need one switch - print a blanking plate! It press-fits into the front panel from the rear, and can be just as easily removed for future expansion.
Bottom panel slides in from the rear, and should snap in lightly to the front panel. Look for the indentations in the sides of the bottom panel when installing - only the rear edge is flat.
Take care when pressing in new panel-mount switches. Even PETG will break under sheer stress.
Printing:
Full print with one blanking plate was about 24.5g in weight.