This is a small case designed for the Raspberry Pi 3 A+ and a LM2596 buck converter that is as big as a Raspberry Pi B case.
The design is 100% 3D printed, no screws needed. Both the Raspberry and the LM2596 are secured using printed 'nuts' (friction fit).
I designed it to allow passive cooling. I tested it during a few hours prints using Octopi, and the Raspberry didn't overheat.
I soldered on the input of the LM2596 the 24V from the Ender3 PSU, set the output at 5.10V and soldered 2 cables with DuPont connectors.
To plug the connectors on the right GPIO pins, use https://pinout.xyz/
I printed the case without supports, but the top of USB gap warped a little, however I could plug the USB cable.
If the nuts are too tight, you can use a drill bit or a screw driver to enlarge the hole.
The cover might be tight as well and bend, you can use sand paper get a good fit.
Designed using Fusion 360
I used these models from GRABCAD for measurements, assembly and rendering: