Improved Craftbot+/Craftbot2 Object Fan Duct

Loosely based on the Craftbot+ Fan duct by sepp974, but done from scratch in FreeCAD. Designed for the stock hotend.

Improvements:
- Better fit to fan angles
- Improved airflow in ducts
- More centered around hotend
- Viewport to help see/clean nozzle
- Prints easily with minimal supports (just under center ring needed)

Mounts with (4) M3x10mm screws.

UPDATED 12/10:
- Opened bottom aperture 1mm, raised skirt 1mm to reduce fan backpressure
- Opened side port 4mm to allow better ac