E3D under-carriage fan shroud for Solidoodle

The original worked well most of the time, but lack of airflow from front caused occasional curling of overhangs (esp. with PLA). This is redone in SCAD (with parameters for all major dimensions and angles), adding a rim.

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**Update 3/18/2014** (v2): Improved airflow; easier to remove/attach (still, a ball hex key may come in handy, with the widened air channel). Still, you will need a reasonably powerful fan, as airflow path remains a bit.. funky; YMMV. See instructions for further det