Circular cooling duct Anet A8 (shortend neck)

This is a modification. The original and superb design (for me) is http://www.thingiverse.com/thing:1746284 (by Eléctrico http://www.thingiverse.com/Electrico/about), that is an improvement about this object: http://www.thingiverse.com/thing:1547256

My modifications:
* The neck (height) is reduced 4 mm because my extruder (it's barrel) is shorter than the original.
* A face in the socket is removed to improve the airflow and reduce very significantly the noise.
* Few edges are rounded