Holds the spool more securely onto the shaft/roller of most 30mm diameter spool holders.
Designed to use with flared ends or holders with end stops.
The split design makes it easy to snap on and off. 
Insert one half then press fit the other half and secure them into place. Fits very tightly.
Print two sets for best results. [mount on both sides of the roll] 
I printed with:
PETG
15% infill