Fan duct designed to attach to a 15x20 blower fan for part cooling. I was just playing around with OpenSCAD trying to figure out how to design this... and well, it works really well too.
Rotate it to put the blower section flat against the build plate. Reflect it to change the direction it blows. I'm using 2 of these (1 on each side of the extruder). The curve angle was set to blow the air at the tip of the nozzle.