Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3458819.

REAL filament (a dutch brand) often comes on a cardboard spool. That's fine, but most filament spool holders may damage and therefore weaken the spool. This ring fits (very snugly!) in the spool, making it sturdy and wear-proof.

Removing this ring will most likely damage the spool, so, print a few sets, recycle them when a spool is empty.