Fully tested now. I tested feeding in my hardest (PLA) and softest materials (TPU) and both fed through quite easily. First passes may be a bit rough due to layering. Might need to run a bit of filament through to smooth it out.
This is a rejig of the Prusa Mk3s extruder to use the old filament sensor with a proxy bearing for indirect sensing. It uses the same bearing and a similar design concept as my earlier mod. https://www.thingiverse.com/thing:3223513. It's easier to print though, as the bearing container (which is the most challenging part) is now tiny and can be reprinted easily if you need to tweak tolerances.
You need a M3 x 6mm screw (with a low profile head) to hold the bearing in place, an M3 x 8mm screw to hold the filament sensor in place and the usual assortment of Prusa 10 and 12mm screws.
You can actually use any screw you like for the bearing, so long as it's an M3, less than 9.3mm overall, with a head no thicker than ~2mm (aim for 1.6mm). And M3 x 6mm is 8mm long and should be ideal.
To assemble, screw the sensor to its carrier, and attach the carrier to the extruder body. Drop the bearing into the bearing carrier (clean out any fluff and rubbish first) and fasten in place with the 3mm screw.
You'll need to reprint parts to use the newer on/off sensor in a standard Mk3s configuration, but these parts should work fine with the MMU2s.
I've included 3 top caps, a standard one, one for an M10 reverse bowden based on this https://www.thingiverse.com/thing:3425397 by Wardy89, and one designed for use with a rivnut as a coupler.
Lower heatsink hole shrunk to dimensions of R3 extruder
E3D clamp shrunk to dimensions of R3 extruder
Cable route for sensor changed to work with older filament sensor, and to make it easier to rout cable for the MMU
Some clearances around the filament sensor improved.
Re-added the window into the extruder on the left side so I can see the gears.
Loosened the filament sensor tolerances.
Modified the top plate with coupler to work with my opening idler door mod.
Added nut pockets for 3 of the 4 screws of the Noctua fan. There's space for a brass insert, but not for a nut in the last corner.
Added a small key tab that links the floating end of the front cover plate into the extruder body by the Pinda mount. This is to reinforce that floating end as it sits near right next to the hotend and is subject to significant radiant heat.