The best model I found for the 3.5" Adafruit touchscreen I'll be using is by designer Tronnic. It mounts to the lower-left aluminum extrusion on the Ender 3, resulting in a symmetrical addition that looks like it came from the factory.

Download the model from Thingiverse and print the following:

QTY File 1 0_Ender3_Touchscreen_Holder_Base.stl 1 1_2_Ender3_Touchscreen_Pi_Case_no_letters.stl 1 2_1_Ender3_Touchscreen_Pi_Back_40mm_Fan.stl or 2_Ender3_Touchscreen_Pi_Back.stl* 2 3_1_Ender3_Touchscreen_Pi_Clamp.stl 1 5_Ender3_Touchscreen_Support.stl 1 6_Ender3_Camera_Bracket

* If you decide to add a fan to your enclosure, I recommend printing the 40mm fan back. Otherwise, print the blank back.

Raspberry Pi 4

If you're using the Raspberry Pi 4 for OctoPrint, be sure to print this remix as well.

Print settings

For the main holder base/bracket, I recommend printing at least 50% infill. If you're using Cura, I recommend the "Gyroid" infill pattern. I printed with 15% "Cubic" infill originally, and the weight from the Pi and fan caused slight sagging.

If you like perfect symmetry, I recommend you also print this separate screen surround to border the original Ender 3 screen.