This is a shelf designed to hold the Raspberry Pi 3 Model B+ with a case but, it should fit other models of similar size as well. It is designed to go on the left side of the base and can be slid into position after removing the front plastic cover.
The author remixed this model.
The original model was designed for a M4 screw and M4 T-Nut to hold the part to the printer. I did not have this hardware at the time so, I decided to add the rail that slides into the front left of the Ender 3 when you remove the plastic cover.