These switches where handed out for free in Australia (they can still be purchased). They basically turn off a power board when your computer is turned off allowing monitors, printers, speakers and so on to be turned off.

I made this by using photogrammetry on the plug.

This stl model isn't what I would consider finished and has some issues that anyone wanted to use it will probably need to fix. But it is working for me without major issues and I'm not intending to work on it anymore.

Notably the section in front of the circular plug hole need to be removed to the front of the opening or there is no way to insert the plug. I just clipped them out after printing. But at least putting a blocker cube with the right dimensions would be better. 

Some of the bolt/screw holes might be hard to access so maybe making sure there is clearance for your tool is a good idea, you will need a long shafted driver for your bolts/screws of choice. I was using hex bolts and didn't have one so I ended up using shaft extenders which I had to assemble half in the plug by putting it inside then connecting the handle to the outside since the shaft extenders wouldn't fix through the hex holes.

I used wood thread inserts for a mounting point in the desk. And I think 4xM4 bolts? But some appropriately sized self tapping screws would probably work too.

I also included a drill jig for hole spacing and a photogrametry model of the plug (It has a circular protrusion you will need to remove from the thing that was holding it while photographing it though).