The original by Mike DaSilva was almost exactly what I was looking for. I'm using it as a plug for one side of the Bambu Lab Over-Engineered Y-Splitter (https://www.printables.com/model/298958-bambu-lab-over-engineered-y-splitter).

Added an inset for a 6mm x 3mm magnet so I could just stow it easily on the back of my printer when I remove it to feed in an external spool. Also enlarged the bevel at the base for a little extra (probably unnecessary) strength.