This will fit snugly onto the 30mm spindle of the S1 PRO and provides a 55mm cylinder to slot your spools onto. (Note, if your spindle does not rotate then this will not help you. Only use on rotating spindles).

The advantage of fitting this is that your extruder won't need to work as hard to pull the filament off the spool and it will unwind more smoothly without tension building up and releasing, which can be a cause of horizontal lines in your prints.
After hours spent 'tweaking' the printer and seeing no improvement, this instantly made a significant difference, and I hope it can do the same for you.

You'll notice it's not a true round cylinder but is in fact a 20 (ish) sided shape.
That's how it's meant to be. It enables a nice push fit onto the spindle and helps the spool grip onto it so your spindle bearings do what they are meant to (rather than the spool slipping around the adapter)

Print 2 of the rings. These slide over the cylinder with a satisfying 'zip'.
Put one on the rear, then pop the other one on after your spool to prevent it falling off.
(To be honest, I ran several prints without the rings fitted and the spool didn't come off, but just to be safe!)