A quick design of a tube serving as a kind of “bearing”, to reduce friction between cardboard filament spool and the stock Prusa's holder.

The idea is to avoid micro dust from cardboard spools and making it bit easier for the extruder to pull the filament.

The part was made a little shorter to allow gripping into the holder for convenience of the assembly. I've printed mine in PLA.

Note: this mod is useless for plastic filament spools. You can see the tube rotating on the holder when cardboard spool is used, but remains stationary with a plastic spool.