Just a little cover for the front y axis idler pulley.
Print it with supports ( for the MGN12 rail slot).
Additional M3 T-nut with screw is needed to fix it on the extrusion.
To be used with the original mgn12h linear rail mod from the link below :
https://www.printables.com/model/79281-creality-ender-3-y-axis-mgn12h-linear-rail-mod-new
The author remixed this model.