Puddlefist designed a great spool holder mount that was the inspiration for this design.
As Puddlefist stated, the 150mm threaded rod that comes with the Anet A8 spool holder is not quite long enough. A longer non-threaded rod is probably best, but this will use what you already have and the rod cannot slip out with normal use.
I would print this at the highest resolution so that the threads come out clean. Also, make sure to print with the flat side down. It is designed to print with little or no support, but your slicer/printer may make them necessary. Also, i printed on a raft, but if you have good adhesion, you should not need that either.