This was designed for use on a original Prusa I3 Mk2 using EndlessParts.com Low Filament Sensor. This filament guide has two guide holes, one for when using the sensor and another for when it is not needed. The sensor attaches to the bottom of the guide using the attached VHB tape. I used a couple of screws as well, just because.
*UPDATE
I updated the print as I realized that printing a tall piece will cause the extruder to hit the guide.
prusa_i3_filament_guide.stl | 138.1KB |