This is a proximity sensor mount for MK8 extruder fan.
The design should be adaptable to different fan and sensor sizes as well as sensor positions.
The square mount plate is attached to the fan with the 4 fan screws already available on MK8. It can be attached in front or behind the fan.
The bridge connects the plate to a circular sensor hold. The bridge is made of two perpendicular legs whose lengths can be changed individually to set the ideal offset of the sensor from the mounting plate.
My sensor is LJ18A3-8-Z/BX having 18mm diameter and it comes with 24mm nuts (27mm outer diameter) and 30mm mounting fixtures. I mounted it without the fixtures, using just nuts.
My extruder's M3x40 screws were too short with this extra mount, so I used M3x50 instead; I set 10mm thickness for the mounting plate to compensate the extra thickness.
Practicing modeling in OpenSCAD