This is a remix of the great print by lrpena here: https://www.thingiverse.com/thing:2887348
No complaints about that file, it's great: it provides a perfect, snug fit for your Raspberry Pi 3 with style and good ventilation.
I wanted a way to mount this without hardware, and without using those little "T-nuts". This is my solution: this print mounts into the aluminum extrusion grooves of the Ender 3 or 3 pro, no additional hardware, no t-nuts required.
To install: take out the 3 screws in the box holding the Ender 3's main board, slide the end-cap out of the way, slip this print in the front and then replace the 3 screws.
Hope this proves useful to people. Enjoy.
I printed the main sleeve up on its end--long direction pointing up in z-axis--to minimize supports. I might have gotten away without supports, but haven't tried that. The only part that (might?) need support is the arrow-clips (that go into the Ender 3's aluminum extrusion) toward the top. I used Cura's experimental "tree support" just on that part and it worked perfectly.
The end cap prints flat with no support.
I turned my extruder temp up a few degrees to make sure it was a nice strong part.