This is meant to hold an LED strip in order to light up your build plate for your Ender 3! It can in fact be printed on the Ender 3 when oriented diagonally. I suggest printing with supports for the led channel. The second model has slots for microSD, standard SD cards, and USB devices. This is designed to have a snug fit in the extrusion slots so that it is very sturdy. To install, simply remove the two bolts on the top right of the crossbar and slide it in!
Please post photos of your print! :)