Inspired by the "Sisyphus Auto-Rewind Spool Holder" made by Vincent Groenhuis I thought it would be much better to replace the original spoolholder with one that works based on gravity as well.
Assembly help: https://youtu.be/r2fYrUZqIkA
The main issue this spool holder solves is the tangled filament on full spools during backwards filament transport. With this spoolholder this will not happen anymore. On top, due of the position of this filament guide the issue of not being able to use full Z of your Prusa was solved as well.
The spoolholder works with every spool I was able to get my hands on. The cones left and right fix the spool no matter of the inner diameter. Even the "fat" Prusa spools are working, since you can adjust to the width of the spool.
Just print the bolt, lower clamp and filament guide one times each, all other parts two times. For assembly you will need 4 screws M3x20mm.
Mount lower clamp and filament guide on the two shanks a shown in the picture. Mount it on top of your printer. Now put the bolt through the spools hole. Secure it from both sides using the cones, the flat side pointing away from the spool. Try to fix the spool centered on the bolt. Now add the gears leaft and right, the flat side showing to the spool. Add the nuts outside of each gear. Place the spool on top of your spool holder as shown in the pictures. Feed the filament through the hole of the filament guide into your extruder.
I printed all parts using DasFilament.de PLA filament, 2 perimeters, 4 on bottom and top. 0.2 mm and 15% cubic infill, except the bolt. The bolt was printed as described above, but changed perimters to 4 and infill to 50%.
Changed shanks to use less filament, speed up print time, and avoid the noise when the gear leaves the teeth area of the shank
print as described, but add more infill and perimeters to the bolt.