I wanted a spool holder that would hold a varying size of spools and didn't require any extra hardware other than what came with my printer. Here is what I came up with.
Designed for the standard 8mm threaded rod that came with my A6. Parabolic caps, and a threaded rod makes it adjustable to suit most spools. The threaded rod is designed to "lock" into one cap preventing it from going farther than need be. There are 2 options for the rod: Short stops at the inner threads of the caps on a standard 1kg spool. Long extends to the outer perimeter of the caps flush with the face. I would recommend the "Long".
It has been working very well and thought I would share.