Neptune 4 Plus Spool Holder with various lengths. I measured the original spool holder that came with my printer, modeled it in FreeCAD, and made both shorter and longer versions.

70mm = 1kg Spool
90mm = Original Length
120mm = 2/3kg Spool

In the future, I may make a thicker version for the 3kg spool (Done 06/16/2024). I'll upload the FreeCAD file upon request.

Print Settings: Now is a great time to use the “Variable Layer Height” setting so that the threads are printed at less than 0.2mm. Other than the threads, it doesn't matter too much.