I made a low profile spool holder with parts who where laying around.
It works well with all the spools I have, from standard and cardboards ones and smaller like taulmann and filaflex.
Uploaded here with source files if anyone is interested:
I made one similar a while ago but I love the spring idea for easy change of the upper idlers. Going to have to steal that one 
Seriously, this is probably the smartest solution I’ve seen so far!
Beautiful design!