I needed to serve (wrap with thread) my new bowstring. This can be done with a loose roll of thread, but it's much easier with a jig to hold the thread in tension. You can buy these jigs online for $10-20, but I'm impatient. Instead I designed and printed this one. It requires a 5/16 inch x 2.5 inch bolt and nut to hold the thread spool in place.

Tension on the spool comes from the jig flexing around the spool, and is controlled by hand tightening or loosening the nut. The nut does tend to loosen in use, so before I use it again I'll probably get a little foam washer to slide under it to make it less prone to unintentional bumping. A Nylon locknut could be a better solution but it would require a wrench to tighten and might break the thin plastic that holds the head of the bolt.

This was a 20 minute project and worked great. The thread I used is Nylon B-50 on a thread-sized spool made by BCY Fibers and sold by Sportsman's Warehouse.