Just got my Prusa and after a few prints, the spool holder that came with it is really bugging me. This one that I designed has a 13.0 cm rod (11.0cm functioning when factoring in arm overlap) however, was designed to add other as the rod attaches on each side in order to keep a high amount of structural support for each filament mounted.
The rod is made up of a series of flat edges to improve friction so that the spool does not unwind during a print (which is my main issue with the original).
The main frame and the piece that connects the Prusa frame are two different pieces that can be glued together and are supported by circular pegs. They are printed separately to fit on the print bed as well as to be able to be converted to fit on top of the Ikea Lack Enclosure modification that I am currently building.