Simple 2-peice jig spool. Wind the serving onto the spool then fit the spool into the jig.

Created as a symmetrical half to avoid needing supports. Print two and snap them together. They snap pretty firmly together, but this will depend a bit on printer resolution, so superglue may or may not be required.

Fusion 360 export thrown in for good measure, but it's pretty basic.

Update: Replaced the original with a wide and narrow version. The narrow version is the "same" but with 20mm less space for thread. The reason is that there is a notable difference between the tension when the thread is playing off the center vs. the edge of the spool. The narrower spool will, of course, require more reloading, but should also yield a more consistent tension, especially with cheap serving jigs.