This is a remix of the popular Ender 3 v2 front electronics enclosure by pedrolamas, but modified to become a back enclosure for the Ender 3 pro.

Notes:

1. Because there's such a huge variety of electronics (mainboard, SBC, CAN, buck converter, etc) these days, this enclosure is completely “blank” and free of any mounting points. The idea is that you can print mounting brackets for your particular components  and attach them in place wherever you need with double-sided tape. This also allows easy changes to the electronics without printing a new enclosure.
2. The top panels come in 2 versions: one with a small cutout and one with a large cutout, which also has mounting points for a 10x11mm open drag chain.
3. The provided top panel STLs are for the right. Just mirror them in your slicer to make a left panel.
4. The top panel STLs have two objects in them: one is the outside and one is the middle. The honeycomb mesh effect is produced by splitting the STL into its constituent objects in the slicer and printing the middle part with 0 top/bottom layers and honeycomb infill.
5. Assembly requires 8 m3x6 screws and 8 m3x4mm heat-set inserts
6. This enclosure is compatible with the stock Y motor mount (you may need to turn the motor so the cable exits at the back), but not the Y endstop mount. I use sensorless homing, but a different Y endstop mount might also work.