I wasn't too happy about the original holder that comes with the Prusa i3 Mk2, since you have to move the holders to accommodate different width spools of filament. So I designed a tube that fits between the holders. The tube is 90mm long, so it fits Prusa spools or narrower.
v2 is designed with an oversized internal diameter (0.5 mm clearance), which gives a nice fit on the Prusa holder arms, as well as chamfered edges for easier insertion. It also has a 3 mm smaller outer diameter to keep spools from trying to roll of it (the outside diameter of the Prusa holders are 25mm in diameter).
v3 is a longer version, 100mm, which clears the 90mm Prusa spools nicely.
Print it upright with a 3 mm brim since it's very tall and narrow!
The filament guide is this thing: http://www.thingiverse.com/thing:1708213
The print was a bit slow, so I actually printed it at 150% speed (from the original Slic3r settings that Prusa ships for PLA)