I couldn't find a filament runout sensor that uses the endstops of my Ender and that fits bowden tubes on each side. So here is my take of one.

You will need an Ender endstop and a short piece of bowden tubing (4mm in diameter) as well as two short M3 screws. I used the ones that held the endstop. The screws will tap screw into the plastic.

IMPORTANT:
In order to use auto unload when the sensor is triggered you will have to bend the end of the metal arm so that the button is pushed down when the filament unloads. Better test this before installing the sensor.