I had this existing built-in shelving unit in my closet and decided to use it for filament storage.  I looked, but couldn't find any existing spool rack brackets that would work for this space so I decided to design this myself.  The rods are 1/2" metal conduit and they fit perfectly into these brackets.  

Printed in regular PLA with 4 wall loops for extra strength, no brim, and no supports.