This print was inspired by a few similar models, but none of the ones I tried worked all that well. In particular, they all had one or several of these problems:

• Correct threads alignments. This knob screws on and off with ease, whereas I've had problems with much too tight fit on two of the others I've tried.
• Big enough sides for fitting all of my spools. For example, the eSun filament spools has a slightly wider diameter hole.
• Easy to grip handles for both sides of the holder, not just the outside
• Easy to print with no supports

Updates

• April 15, 2023: This model also fits nicely on the MK4.
• Dec 28th, 2022: Added the Fusion 360 file.
• Oct 8th, 2022: Added the .step files.
• Feb 21st 2022: I made some very minor adjustments to the fillet on the knob and the tube, for less bridging.

Printing

This should print easily without supports, flat on the knobs (as in the .3mf file). I've printed with 0.4mm nozzle, 0.20mm layer height, 3 perimeters, and 15% infill. I've been using PETG, but I don't think material should matter (I just wanted my colors match the rest of my two Prusa MK3S printers).

Attaching and using

Twist off the existing spool holders, and inset the holder through the printed tube (see pictures). Reattach this back on the holder frame, and it should stay there permanently. Now you just put the spool over the tube, and screw on the knob from the outside; it should slide easily over the end butt, but the printed tube should not come off.

Make sure the spool is properly centered, you may have to help it by lifting it up while tightening the knob. This should make for a snug fit, the printed tube will spin over the existing spool holder. Meaning, the spool should not spin on this printed attachment.

Questions and feedback?

I hope this works for many spools, but if you have problems, or feedback or ideas for improvements, let me know. I can't take credit for this idea of course, I just improved upon existing designs. But it is a completely new design from scratch, and I'd be happy to share the Fusion 360 file if someone cares.