Summary

A simple rotating yarn holder for 2 spools, using printed or metal 6004 bearings

Holders have 2 versions - normal (15mm in diameter) and thin (10mm in diameter), use whatever you want.

Printing: print base and 2 holders. Holders will need supports under the bearing slots!
Settings: whatever you normally use, 10-15% infill and 0.2-0.3 layer height, any hard material. Spool holder upper parts are modeled to be hollow, as infill feels unnessesary and greatly extends print time

You will need to print a pair of 6004 bearings. Ones that i used:
https://www.thingiverse.com/thing:4982500
Try not to use the wide clearance version, or the holders will have a lot of wobble.

Assembly: apply a generous amount of light lubricant to bearings and the bottoms&sides of holder pieces (sprayed slilcon lube works great), then put everything together. You may want to add small rubber feet to prevent sliding. Parts should have a snug fit, but holders can still be detatched from the base with a light pull. Holders should rotate freely without touching the base - there should only be friction in the bearings. Adjust scale slightly to get a tighter press fit if you want the whole thing to be in one piece.

Optional: the base has a small rim so that the bearings&the holders are elevated sligthly to prevent friction against the base. If using printed bearings with large tolerances, the holders can tilt and rub against the base. Print a spacer piece and put it over the existing rim to raise the holders a bit higher.

STEP file included if you want to redesign for different bearings. Common 608's were too small for this, and 6004 seemed to be the biggest ones that still have a lot of printable variants available. If you want minimal friction and noise, using cheap and large washing machine bearings might be a good option, like rather huge 6205 that i could find for 2$ a piece online