I bought a Creality Ender 3 Filament Runout Sensor for my Ender 3 Pro. Despite being an official product, I didn't see many printable mounts/brackets for it, only for other variants which mostly had 2 bolt holes. This sensor, despite being only few grams, has 4 bolt holes.

The sensor comes with a metal bracket to mount it horizontally right beside the extruder, but this causes a sharp angle as the filament turns upwards towards the spool. I designed this simple bracket to mount the sensor on the spool holder, in the natural path of the filament. It also holds the filament off to the side a bit to ease the angle at the extruder.

This bracket is compatible with the Ender 3 and Ender 3 Pro, as it requires the old-style filament holder. You will need 2 or 4 M3 bolts with countersunk heads to attach the sensor to this bracket. The bolts bite into the plastic you don't even really need nuts on the rear.

This is my first CAD design so it is functional rather than beautiful, but it seems to work OK. There is currently no cable management so I just zip tie the sensor cable to the arm, and run it down the groove on the rear of the frame to the motherboard.