GeeeTech i3 Pro B Air Duct

I've designed this air duct for a 50mm radial fan on my GEEETech I3 Pro B after using XDr4g0nX's fan shroud for quite a while (thanks for the design!).

However, I think my approach has several advantages:
* The base and the throat are much more slender. So it doesn't conceal so much of your prints and it does not hit the glass clamps.
* Its height is adjustable in the range of 3.5 mm which may help to avoid some obstacles.
* The ring has a lot more holes which are directed downwards inste