Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3595011.
I wanted to add a Raspberry Pi running Octopi (OctoPrint) to run my Ender 3 Pro. It can be see in the pictures in crowser's remade case (https://www.thingiverse.com/make:573845). I wanted the pi to run off the Ender 3's PSU, and went for the USB powered route instead of hard wiring a buck converter.
Here is the buck converter I used - https://www.amazon.com/gp/product/B07121W7Q4/ Others would probably work just as well, I just looked for 3A (5V) USB out that could use 24V input. If you don't get 3A output, the pi will probably complain about being underpowered (mine complains if I use a 2.1 power supply). I wired the buck converter to connect directly to one of the unused 24V pairs on the PSU. The USB power connection is at the back of the PSU case.
I created a test print to make sure my mounting mechanism for the buck converter worked (buck converter mount). You could use this to quickly test or adapt for a different buck converter. Once this seemed to work, I incorporated it into the PSU case. It's not perfect. One of the clamp prongs snapped on my full print, but I made it work (didn't want to print the whole thing again).
For the PSU case, I pretty much followed the design of the original. I put the buck converter mount on the bottom, at the back. to make sure there was enough room, I deepened the whole case by 5 mm. There didn't seem to be any way to print it without supports, unless I changed the design significantly. I used Cura's Conical Support (experimental), which seemed to work well.
I included sketchup files if you want to edit it.