This is yet another low-profile PSU conversion for the Ender 3.
It features a Meanwell PSU tucked away behind the printer and doesn't protrude out the back or above the base 40x40 extrusion.

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You will be rewiring cables which are connected to mains voltage in the end!
If you don't know enough about electronics and don't feel comfortable doing it, ask a friend who knows this stuff for help.
You will be opening the PSU! If you touch the wrong part you'll get shocked!
Ideally wait a few days with the printer unplugged so the PSU is safe to handle.
I'm not liable for any damage, fires or electrocutions that occur!
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The power switch and cable plug are relocated under the screen.
The PSU is relocated to under the rear of the printer, where the Y-axis motor is.

You will need to rewire and crimp new cables, as the stock ones aren't long enough.
You will also need to drill holes in the 40x40 aluminum extrusion.

Parts:

• M3 screws
• M3 T-Nuts (metal or printed. I used these: https://www.printables.com/model/129445-t-slot-nut-m3-for-alu-2020-v-slot)
• 16AWG/1.3mm cables
• spade connectors and blade connectors

Printing:

I printed all the parts with 0.28 layer height and 15% gyroid infill.
Orientation can be seen in the pictures.
Supports are only needed for v-slot-2 and fan-cover.
I used PETG-CF from Bambu Lab, but any filament should work.

Assembly:

The whole conversion should be doable with the printer mostly assembled.
Drilling the holes will be tricky, but I think it's doable.
I needed to square up my printer anyway, so I disassembled most of it.

This is not the best assembly guide, but here we go:

• Unplug the printer!
• (Optionally wait a few days so the PSU is safe to handle and be worked on)
• Take pictures of the wiring so you know what goes where when assembling.
• Fully disconnect all the wires that run to/from the PSU. You won't be needing these anymore.
• Drill holes in the V-slot (location indicated in the pictures). I used a 4.5mm drill bit and made the holes close to each other. I combined them into one oval hole by wiggling the drill bit around a bit. Insert the cable protector into the slot.
• Take off the display holder.
• Feed new cables through the drilled slot until they come out the front where the display holder was. Leave 15cm on both ends of the cable so you have enough to work with.
• Install 8 M3 threaded inserts into the box.
• Drill holes into the display holder.
• Place the spacer between the display holder and the 40x40 extrusion and screw in the display holder. Take care not to squish the cables.
• Screw in the box.
• Run new cables from the mainboard to where you drilled the holes. These will connect the PSU to the mainboard. Connect them to the mainboard and leave +15cm of cable from the holes.
• Take off the PSU cover and disconnect the fan.
• Bolt the PSU to the middle 40x40 extrusion in the bottom V-slot with two T-nuts and the M3_spacer. These are there so the cables don't get squished.
• Slide in v-slot-1 in the bottom V-slot where to terminal connectors are.
• Place the fan inside the PSU where it will be in the end and connect it. Don't screw it into the cover yet!
• Slide in the PSU cover. This will be fiddly.
• Once the cover is on try to line up the holes of the fan with the holes in the cover. Once lined up screw them in without the fan cover for now. This will be even more fiddly than the step before. But you can't slide the cover in with the fan already screwed to the cover, so there is no other way.
• Now you can shorten and crimp all the wires, as all critical parts should be in their final position.
• Connect all the crimped wires to the switch part and the PSU. Reference the photos you took earlier.
• Tidy up the cables, close the box at the front and slide in v-slot-2 at the back.
• Unscrew one of the screws holding the PSU fan. Attach the fan cover and screw in the one screw. Repeat for the second screw. If you unscrew both the fans at the same time they will fall into the PSU and the fishing game will begin anew!
• You can optionally attach the printed clip to the PSU and v-slot-1 to stiffen up the build. (see pictures)
• This should be all.

I've made a video where I give a brief overview of the finished build. This is no assembly guide, just a brief showcase and pointing out a few things.
Excuse the overly unenthusiastic voice-over, but the originally recorded audio was even worse^^ As you can see and hear I am no video creator.

If there are any questions or you need help, just write a comment.