I purchased some generic, cheap “made in China” filament sensors a long, long time ago. These sensors have no real way of mounting directly to 2020 extrusion, and the internals are really oddly shaped and sized so I couldn't use an existing mount available here.

So I created this small elbow-shaped mount. Simply install two M3 inserts, attach the filament sensor to the mount using two M3x16mm screws and install the entire thing to the printer using a M3x8mm screw and T-Nut. 

I purposefully designed this to exit at 20 degrees so that the PTFE tube enters the chamber at an angle.

I printed this using PLA, 0.2mm layers, 2 walls with 10% infill.