I have had innumerable spool holder designs on my taz fleet (my own designs and downloaded versions), and I've finally stopped after this design. It's a single print, solid as heck so it can be made with any material, and stays out of the way with tall prints. Handles any spool thickness, and adds some rigidity to the frame.
Requires m5 t-nuts (I prefer drop in) and m5 x 12 or 15 bolts.
I measured the space between upper rails, and how high the spool needed to be. I wanted a zero support, single print part. Hogged out some material for strength and material savings.