I printed out the Filament Spool Shelf System by MikeGTP and really love it. The only thing I didn't like is that the spools could be pushed right through each shelf (totally a ME issue and not the model), so I designed this friction fit clip as a guide/stop for the spool. The clip fits on the very back of the shelf and acts as a guide for the spool to sit, as well as a stop to prevent the spool from being pushed through too far.

NOTE:  This is ONLY the clip.  The shelf is a separate piece you need to print out which is part of the model linked above.