NOTE: This is an update to my [original rear plug PSU design] (https://www.thingiverse.com/thing:3180564)
Although the original design was functional and served it's purpose, several users reported that it tends to warp during print. Although I have not completely eliminated warping with this design, I have minimized it to the point that it's not an issue (you might notice the bottom of the case is a fraction of a millimeter not level, but the bottom component will easily flex to fit this small error).
I wanted to use a Mean Well LRS-350 power supply with my Bear upgraded MK3, so I designed the case available here: https://www.thingiverse.com/thing:3111868
Some people asked about a version with the plug coming out the rear of the case, so I created this design. The Prusa MK3 plug has soldered directly to it a "power panic" board. This is a small PCB with a circuit that detects that mains power has failed and notifies the printer controller before the 24V supply from the PSU also fails so that the state of any in-progress print job can be saved. Because this board is soldered directly to the terminals of the power (IEC) socket, the layout of the case is somewhat constrained. This is why there is a large bulge around the socket.
Like the [side-plug version of my Bear PSU case] (https://www.thingiverse.com/thing:3111868), it attaches to the frame in a manner similar to the Prusa PSU with the mounts in the Bear print list, however the "upper PSU mounts" are different (STL is included), so make sure you print those instead. The "lower PSU mount" is the same as that used for the Prusa PSU.
Like the Prusa PSU case, and unlike my other design, this case has a removable bottom. I connect the mains Ground/Neutral/Load wires and 24V/GND wires to the PSU outside of the case, and then slide everything together. Then I plug the mains wires into the plug and the switch using a pair of needlenose pliers.
To attach the case to the PSU you will need 3 M4x8 flathead screws. If you want to use longer screws, check carefully that the screws on the back of the PSU are not contacting the circuit board. The brackets mount with M4x8 button head screws, just as with the original Bear parts for the Prusa PSU. Flat head can be substituted if that's all you have.
To attach the bottom to the case, you will need 4 M3 square nuts (DIN 562), and 4 M3x8 flathead screws. Both of these can be sourced from the Prusa PSU cover.
The case has a hole in the left hand side of the front. There is an additional component you can print, Voltage_Adjust_Guide.stl. This fits into this hole and provides a guide for a small screwdriver if you need to adjust the PSU output voltage after installation (the guide is there so that your screwdriver doesn't wander off target and short something out).
As with all my designs, the Fusion 360 source file is provided so that you can easily do any customization that you find useful.
EDIT (2019-01-27): Fixed an issue where the "foot" was inadvertently moved too far forward, and so would not line up with the frame bracket.