12/24/2018 Added the Bearing TPU seath from the MMU2 design because it improves the sensor performance dramatically. Also changed the sliders to round, converting them into rollers instead of sliders, I guess

12/3/2018 I am including the STP file of the assembly so anybody can easily adapt the housing to the bearing and magnet that they have handy.

This is an attempt to make the PRUSA MK3 filament sensor work safely, reliably, and also with any type of filament, even if it is transparent.
The attempt is (as now) 100% successful (with just one beta tester, that is me, so feedback from other people is welcome), and it is aimed at ending with the skipped layers of the MMU2.
The sensor housing is intended to be assembled on top of the extruder body and use the stock sensor cover of the MK3. Assembly time is less than one minute.
No firmware modifications needed. because although the sensor is reading "upwards", it's also rotated 180º

To build the filament sensor housing you will need:
1- One of the bearings that PRUSA includes with the MMU2 spool holders (∅22 ∅8 x 7.5 mm)
2- A magnet- The magnet housing in the plastic part is diameter 11mm x 2 mm length. I used a Neodymium magnet diam 10 x 1 bought in Aliexpress- Costs barely nothing.
3- Two M3 x 40 bolts. I used two that came with the MK3 as spares
4- One bolt M3 x 5 to hold the sensor in place.I used a Nylon6 slotted bolt because it was easier to cut to lenght.

11/28/2018
This part is intended to be used with the MK3. If you are using the MMU2 you might consider using the PRUSA MK3 MMU2 EXTRUDER BODY W/ FILAMENT SENSOR INDIRECT READING MOD (https://www.thingiverse.com/thing:3245715) that I have just uploaded. The sensor housing is included in the extruder's body so filament loads do not have to cross through so many parts on their way to the extruder. It looks way more neat and elegant...