Ever have the problem that wire just spills off its spool? I had this especially with some spools of wire I ordered from Adafruit, the 8-color "wire wrap" wire assortment (75x30) and the silicone-covered stranded wire (30x25).

Enter these holders: Feed the wire out the little slot, then snap the whole spool in. Snug, but still ready to access at a moment's notice.

If you have a spool of a different size, simply measure its outer diameter and overall height and enter them in the spreadsheet tab in FreeCAD to regenerate the design, then fillet the top as desired (due to the topological naming problem, I had trouble getting a top fillet that didn't interfere with sizing the holder)

Note: I work with Adafruit but made this design on my own time.