This is a fan duct I designed based of the Tusk Part cooling fan idea by SublimeLayers. I modified it an built a version strongly inspired by the one from cdoe.
This duct is using a 40mm blower fan and 2 pieces of 4mm (OD) brass tubing. It's working great so far - maybe I will make a different version vor a stronger 5015 blower fan.
I also provided the f3d and stp file.
Turn on "thin wall detection" for this print. The top part, which "seals" the fan and duct is only 0.45mm thick (or thin).