Update: Rev2 files have more clearance at the top of the motor so the extruder arm upgrade doesn't hit it as easily. Also, there are some weight-saving cutouts that don't significantly do anything but look kinda cool.
A replacement Filament Extruder Guide for the Rep2. It's lighter and has better airflow.
The main thing is that it allows air to exit out the left side of the heat sink. The material that was previously there was not adding much to the strength of the piece, so I removed it to allow for more airflow. I also reduced the size of the top part, since the original part was unnecessarily large.
The Solidworks file is included for download.
While this design is original, and not a derivative of anyone else's work. I did copy the rough shape of the top piece of DHedrick's version of (http://www.thingiverse.com/thing:113595).
Print it with 0.2 mm layers, 25% infill, 3 extra shells. Then remove the mouse-ears and file the rough spots off. Before installing, use the fan-screws to pre-tap the holes. That's it!