Many of the filament guides I've seen require either more vitamins (bolts/nuts/rods), or the removal of the frame nuts for installation. I had a heck of a time squaring and tightening my frame, so I didn't want to have to take it apart again.
This filament guide is a snap-in one designed for the pitch of the threaded rods on the original Mendel (58.89mm). I don't know if the Prusa has a different pitch, but this guide should work even if it is up to about 5-7mm narrower than the Mendel (obviously wider won't work so well without modification).
I printed one out of PLA and it snaps in nice and snug. ABS may have different stiffness.
I also used the diagonal slot idea from fil's thing http://www.thingiverse.com/thing:11222, but I moved it around to the top like he suggested so that the guide can be installed "either way".
Print, snap-in, install filament (or snap it in, too!).
Skeinforge 35 was giving me STL errors about holes in the triangles, but it seemed to print ok. I have also included the native file (Solidworks) for editing.