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.