Summary

For Prusa Mendel model.
This guide is 3 pieces. A left, a right bracket and a center guide that connects via dovetail joints.Make one of each. It allows the filament to slide back and fourth with the movement of the extruder head.

Instructions

Make one left, one right bracket, and one middle bracket.
Recommend printing with a brim and cleaning it off after. The center guide is rounded with a fillet and prints nicer with the brim.
Be sure to clean parts well and sand the center part to make the fillet smooth.
Sand dovetail joints so they fit nicely (tight but removable)
Optional: In mine, i like the ability to remove the guide without disconnecting my filament from my extruder. So i heated an xacto knife over the stove flame and cut the end of the center bracket right thru the dovetail. See the screen shots.

Wide Filament Guide