This is designed for the Prusa Core One and spools with large middle holes (arbors), this one specifically made for Giant Arm filament (but will work with any filament that uses this exact same widely manufactured spool). Without the attachment, the spool hangs down low and touches the printer (see picture); I don't like this sloppy look.

The outside diameter is 75.35mm and should slide just perfect into the spool arbor; the inside diameter is 47.8mm and leaves just enough room to fit around the Core One's arm.

Tinkercad was used to make this.