This Thing Does Not Work Yet
The V2 design has a small problem. The "fins" that stop the power supply from just sliding through are not supposed to hit the power receptacle or power switch. My actual part doesn't quite match the model I found on Thingiverse, and these parts ended up interfering with the PSU holder fins.
V2.1 may fix this but I haven't had a chance to print it and see.
I am working on making one of these enclosures I found on prusaprinters.org. My modification is that I am making the bottom table an enclosure too, for a different printer. The stack will sit on a larger, flat desktop. The original placement of the PSU holder puts it inside my lower enclosure, so I am moving it to the outside of the table leg on the lower table. Basically the opposite side of the table leg is would have been mounted to. The PSU will still be upside down, but it will be even closer to the point where the wires exit the enclosure.
The is also a right side up version in case you want to try and mount it to one of the legs on the upper table. In this case there's no place to put the PSU until after you've put the enclosure over your printer, might be annoying to deal with if you plan on taking the top on and off a lot. I am not currently planning on using the right side up version, so it will be untested unless someone posts a make.
Filenames that contain "RSU" are for the power supply being Right-Side-Up, oriented the same as when mounted on the frame of the Prusa MK3. Filenames that contain "USD" are for the power supply mounted Up-Side-Down.
The little tabs with screw holes that stick out need supports.
The thing I am remixing includes STEP files for all the parts. I took the STEP file for the PSU holder, loaded it into SolidWorks, and modified it for my purposes. I found this thing on Thingiverse that had SolidWorks files of the Prusa MK3 printer itself, that's where I got the PSU model to use. I found a model of the IKEA Lack Side Table on GrabCAD and used that to make sure everything would fit up as expected.