Features:

• Quieter than original K1 duct (on my machine).
• Optimized using computational fluid dynamics. 
• Similar performance to the stock duct. 
• Relatively easy to print with only a very small support required.
• Looks good (IMO)
• Only 35g of filament when printed with ASA.

Hardware:

• A 3 mm heat insert OR a M3 nut.
• If you are using this for a second auxiliary fan, see Derrick Darrel's dual fan mod. 

Note: if you are interested in the tool head cover in the cover photo, click this link. It cools better, its lighter, and its easy to print. Also, shout out to Derrick for his excellent model. I referenced it for the required dimensions (hole position, size of intake, etc.).

The main benefit of the duct was a happy accident. I only intended to make an easier to print duct than the one included in Derrick's excellent dual fan mod. It just so happens to be quieter than my stock duct and fan.  I am not sure why but some possibilities include:

• Streamlining the interior surface (the stock duct has a sharp ridge at the inlet side and a thicker inlet wall).
• There is a bigger gap between the back of the duct and the side panel.
• Its a single piece.
• It fits more snuggly into the fan 

Either way, my auxiliary fan is quieter now.

This design still has the same shortcomings as the stock duct. Specifically, the entire front of build plate and the right side corners (near the aux fan) lack air flow. There is a good chance that overall air flow could be improved with some tweaking as well. I will be looking into these possibilities at a later date. 

Print settings:

• Model is oriented in the recommended print orientation.
• Default support with “on bed plate only” selected.
• You may wish to support the top side of inlet with a tree support as the bridge is 26 mm. I tried with and without supports and the results were very similar. If you do add the support here, you'll need to manually add or block supports to avoid couple of unnecessary supports.
• Layer height around 0.2 mm. If less (like the 0.15 I used), I recommend “thick bridges” is turned on or to support the top side of the inlet for the 26mm bridge. A 0.2 mm layer height should be fine without “thick bridges” but thinner may cause trouble.
• I used a 0.5 mm wall thickness.

Please leave your feedback and enjoy.

Rick