[Parametric] proximity sensor mount for mk8 fan

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 plat