A spool holder extension that simply plugs into the original one. I designed this to be able to use some wider spools (like from PM) without risking them falling off.
You can use it with or without the nut in the middle, depending on what your spool requires. The extension just clicks in place when inserted all the way through.

It is an extension, not a replacement, for a simple reason: better load capability. The original tube is very rigid, but short. Using an extension that runs through it, I use it to support the extended part, dramatically reducing forces that could cause layer separation on the printed piece if it had a similar overall length. Therefore it should hold multi-kg spools easily.

Creality

Ender 3

No

No

0.2

10% Cubic

Unless your bed adhesion is horrible, no raft or brim is needed. PETG is preferred, but PLA should work too. Simply print in the supplied orientation.
Use at least 1.2mm walls and 1mm top/bottom thickness for rigidity.

No postprocessing needed. Simply slide it into the original spool holder tube from the back until it clicks in place. Remove the original nut first if necessary for your spools.

Designed in FreeCAD, the file is included for potential remixing ;)