Firstly, the Boots spool holder is a fairly awesome design! I love that it is fairly low profile and has a bearing for smooth payout. I was a bit bothered by the fact that the 608 bearing is used under load in a horizontal orientation. While there's not a ton of sideload on the bearing, it is nevertheless not great for the longevity of the roller skate bearing. I also had to print a 2nd base for the holder when I first tried to use it because the small (8mm) peg that the bearing sits on just snapped off when the spool leaned a bit too far one direction.
So I remixed this with a larger platform to hold a thrust bearing (XP51105) which I bought on amazon for $2.67
The bearing sits flat on the base and over the larger diameter column in the middle to keep it centered. A 4mm bolt will thread down into the hole in the center, and if you have a large washer under the head of it you can tighten the bolt to put a tiny bit of "drag" on the holder so your filament doesn't just unwind itself into a pile.