The SeeSaw Spool Holder is rather self-explanitory. In essense, it is a seesaw, and performs just like the one at your favorite childhood playground. When a spool of filament is in use, the front end of the "hanger" is heavier, and it falls forward - when there is no spool of filament, the "hanger" falls back into a closed position due to a counterweight that makes the back end heavier. The bracket that holds the rod up is specifically positioned so that the hanger will catch on the front 2x1, and not rotate out of the out and closed positions. As an added bonus, the "hanger" is able to slide back-and-forth across the shelf, allowing for a spool to sit directly above multiple 3D printers in different locations.
For the rod, I used a wooden dowel, but spare piping could also be used. The size of dowel was .375 in. (about 1 cm).