Spool holder designed to fit under the MINI BASE 
https://www.printables.com/cs/model/63483-prusa-mini-base
for easier unwinding of the filament. Bottom of the spool is now 5mm above the table plane, so it could freely rotate, but it is still located under the base in the appropriate cutouts.

Added few centering rings for some other brands than Prusament filaments.

For bearing use 6mm balls, e.g. airsoft BBs or from steel bearing. 

Print from any material, even PLA works great. It is recommended to glue top part with top_plate part, but it is not necessary.