Shelf system spool mounting bracket, sized for Joel Telling's (3D Printing Nerd) spool storage shelf system.  The brackets pictured were scaled down to my print area (200 x 200), but as you can see they still hold a full spool.

A simple design, print 2 brackets and 1 holder. The holder width can be expanded for oversized spools, and the distance between brackets adopt the size dictated by the holder width.  Gravity locks everything into place once inserted between shelves.

The holder width and bracket height can be changed on the Fusion 360 sketches and the dimensions will track (provided you do't go to small for the constraints).