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