I sloppily threw this together and it worked great. The only complication is trying to get the T-nuts lined up after you position it, since you can only pre-load one side with t-nuts or else you can't fit it in. Roll-in t-nuts would be ideal, if you have them.
Other notes:
I got this working with the following klipper/ratOS config. The sensor pulses every 7mm of filament, and this tells the firmware to expect at least one pulse every 25mm. This means as long as the filament is moving ~30% speed or faster, the sensor will be happy. The margin is primarily to avoid false positive pauses due to slack in the bowden tubes, occasional skipped steps, etc.
[filament_motion_sensor filament_sensor]
detection_length: 25.0
extruder: extruder
switch_pin: PG15
runout_gcode: M600
The author hasn't provided the model origin yet.