After trying out a few wire spool holders on here, I decided to spin my own from scratch because they weren't quite working for my needs.
This is a parametric spool holder and is easily customizable for your own needs. I'm using mine for 5 spools each, but it could be expanded (or contracted) by entering the number of spools and their thicknesses.
The rod is designed to snap in place, but can be removed with a gentle amount of vertical force.
The STL files are for a 5-spool (5x) and 7-spool (7x) holder, each spool with a thickness of 20mm.
Supports are only needed for the rod, and only then if you print it as oriented.
Update Sept. 14, 2018: I made a revision to the OpenSCAD design to give some better tolerance to the mounting point on the holder, as well as copious comments, plus a full assembly view. Model files have been updated to reflect this. And I generated one for 7 spools as well.