I created a filament runout sensor that uses cherry MX switch.

It consists of 2 parts "keycap" and casing. You'll also need any cherry mx style switch and a cable to connect the sensor to the mainboard.

I used tiny drops of super glue in to fix button to the frame. You may also need to sand the keycap round, because the filament does not slide in smoothly otherwise. 

I still haven't completely figured out the mounting for the sensor.