I made this in TinkerCad to fit the 30mm spindle on the Ender S1 Pro. It fits the 56-56mm hole in my Creality plastic spools and will probably fit others.

It's only 5mm thick but fits snuggly (for me) and I've ran several 13 hour prints without issue.
Using this has resulted in significantly better quality by reducing visible horizontal lines.

I printed in standard PLA in high quality (Cura) just to ensure it was true to size but I think it would be fine in normal. I think it works well (in that it's a snug fit but doesn't get totally stuck) because it's not a true circle but has sides (I haven't counted them, maybe 20?) and this was purely a side effect of the cylinder shape in TinkerCad not actually being round!
But once I noticed this in Cura when slicing, I realised it would actually be an advantage.
The holes save a bit of plastic and allow some slight flex.

You can make it taller in your slicer if you wish (I'm going to make a more cylinder shaped version) but it won't have 'wiggle room' the way this flat disc does so it might be a it tight.
Unfortunately you probably can't scale it to change the outer diameter in most slicers as the internal diameter will change.

EDIT - I'm finished the cylinder version which I think is a better option than this if you change your spools more frequently. If you only change them when they run out though, this one works and is cheaper / quicker to print.