Ducted extruder carriage for Rapman

Replacement for the acrylic single-head Rapman extruder carriage.
This design incorporates a fan duct that more effectively cools the area being printed, reducing curling and melting.
Also holds the extruder fastening bolts securely, without them loosening and turning like the standard ones frequently did on my Rapman.

Added July 2014: a couple of modifications!
bkt60a.stl is a bracket that bolts onto the top of the rear end of the extruder carriage that allows a 60mm fan to be mounted and pointed at the top of the hot end. On my printer, this has totally prevented the heatsoak problem causing PLA to jam the hot end.
duct shield.stl is a simple shield that attaches to the main duct at the bottom, shielding the tip of the hot end from the airflow, thus improving temperature stability.