Dual Spool Holder for the Ender 3 Pro (may also work with other models, but I only tested it with the Ender 3 Pro) It uses the existing Spool Holder and replaces the locking ring on the other side.
I printed it in PLA with standard settings (0.2mm layer height). The overhang at the top turned out a bit rough, but not terrible.
The author marked this model as their own original creation.