Cardboard spools from Overture and Polymaker are susceptible to de-spooling particularly when full.  This prevents the filament from hopping over the spool rim.  The guard clips on magnetically for easy attachment and detachment.  Works with the both the original spool holder and my ball-bearing holder.  Come in both a closed and open version.

You will also need ¼" round magnets.  Up to 5 slots are provided.  Use a wrench to press them in.

Even though this does not make direct contact with the spool itself, it does a pretty good job preventing the filament from hopping over the rim and getting tangled up.  But not always.  The primary cause is the bowden tube points up and outward which induces the hopping.

Therefore a filament hook has been added to guide the filament downward and inward.  This is also magnetic and can be moved around.

Edit: Thin filament hooks has been removed, and the wide filament hook is shortened by 5mm to further pull the filament in toward the middle of the spool.