The file 'Ender 3 Spool Holder – Side Mounted 3D Printer Model' is (stl) file type, size is 222.8KB.
Cleaned up and modified the design. Cored out the front and added some ribs to reduce material and add structure.
With 70% infill it's very strong and sturdy.
Check out the original from @cerkit for installation instructions.
I would recommend printing a small piece (about 3-4mm high) to check the fit on your extrusion. It should fit snugly but still be able to slide in from the back of the extrusion behind the Z-motor.
Side-spool-bracket-design.stl | 827.9KB |