Idler bearing for the Ender 3 X rail on the stepper side. Only additional parts required are qty 2 6x10x3mm bearings. The bearing cap is secured by spot welding once the bearings are installed. The mount slides into the right side of the X rail, with the dovetail on the front side of the rail. The idler bearings prevent the teeth on the drive belt from contacting the aluminum extrusion and causing strange vertical patterns in the print. My print was in PLA, but ABS and other materials could also be suitable. I opted for 100% infill, but as this is a low stress item, less infill should also work fine.