The 2kg spool hanging unevenly on my rail was causing minor defects in the print surface, this axle allows the spool to sit centrally on a 10mm rod and spin smoothly.

Print both parts on their sides (the flat side to the bed), this will put the layer lines in the strongest orientation for the use of the final part.
Part A needs supports for the thread, part B can print without.

To assemble you just need to screw it together and thread onto a 10mm rod.
I needed to drill out the inside of the 10mm hole with a 10mm drill bit just to get it to fit, but still spin smoothly.