UPDATE: Added new V2 version, which puts the PTFE holes further away from the buffer to make it easier for fingers to get in there. Haven't printed the new version myself just yet though. :)
UPDATE2: I now use the 1st version on the spool side (filament is more likely to make it in to the correct channel when initially loading), and the 2nd version on the printer side, because it has a few mm more room for fingers. Works great!
I was about to give up on the stock Prusa filament buffer, but couldn't give it up just yet.
This adds extenders that hold M10x1 pass-through PTFE connectors that restrict the filament flow much less than the smaller-threaded PTFE connectors. This also puts all filaments on one side, as well as offering more finger-space and funnels to guide filament into the printer-side.
Print two of these parts; one for the printer-side, and one for the spool-side.
0.20mm layer height worked for me with 4 walls, 4 top and 5 bottom layers.
It fits on all four possible positions, using 2 or 3 screws, screwing each through the extension into the old filament-keeper-screw holes.
Be careful not to cross-thread the M10 connectors. They should fall nicely into the threads.
Combined with pmeyer's improved rollers:
...and my remix of s_kohls' remix of codiac2600's M10 Festo holder for the Prusa spool trays:
...I'm finally not getting filament slipping through the MMU2 on filament load to the extruder.
Nice and smooth and quiet. Thanks to pmeyer, s_kohls, and codiac2600 for the basis to this!