Wall-mounted filament spool rods holds up to five spools. Use 1/2" Sched 40 PVC conduit for the rods. The inserts adapt Prototype Supply reels to the 1/2" PVC (those are a remix of http://www.thingiverse.com/thing:224151 with the inner and outer diameters adjusted).
Width is probably overkill (takes 4+ hours to print on my Printrbot Simple Metal), but ends up plenty strong.
UPDATE: added Sugru to the rod holder cutouts for a more snug fit and to prevent any possibility of the rod being spun by one of the reels. Split a Sugru packet into 2 pieces (one for each side) and molded a thin layer along the entire concave area of the cup. Add just enough thickness and grip to hold onto the rod while still being easily removable.
Print left and right spool holder. I mounted with 1-5/8" exterior primeguard screws straight to studs. Cut PVC to fit. I decided to only go 16" to reduce the load on the PVC and the bracket. Also to reduce the angle at which the filament drops to the printer.