Improved customizable fan duct

thingiverse

I tweaked the original duct generator design by incorporating an adjustable parameter that lets you set the length of the straight inner wall section where fan blades meet the duct wall closest. For single-fan ducts, this measurement can be zero, but for contra-rotating fans like I'm planning to use, it's roughly equal to the vertical space between the two fans, possibly with a slight adjustment factor thrown in. This length is specified in millimeters.