This is a top mounted spool holder for the Robo 3D printer. It uses two 3/4 inch dowel rods -- one to act as a stabilizer (the lower rod) and one to act as the spool hanger. The upper end of the printed piece is open to facilitate easy removal of the dowel to add or remote filament spools. The lower slotted ends fit into the hole on the top of the printer.
I really like Keraynopoylos' design at http://www.thingiverse.com/thing:188820 but I wanted something that could hold multiple spools instead of just one. My first designed used 3/8" dowels but had to switch to 3/4" since my local hardware store only had a single 3/8" rod (and I wanted several) so 3/4" it is.
It goes well with my filament guide which is designed to also sit on a 3/4" dowel. http://www.thingiverse.com/thing:223573
This was created using TinkerCad.
I printed this with 35% infill and 6 layer borders. You probably don't need this much but I wanted to make sure it was nice and strong.