I have designed three versions of fan duct and this is my favortie one. With the least amount of modeling functions been used, the inner wall of this fan duct is designed very smooth, consistant, thin and light. 

Most importantly, I have gonee through three rounds of improvements usinng Simscale to simulate the best fan duct that converge the air towards the nozzle and no elsewhere!

I have not only uploaded the fan duct, but also the whole printhead just in case you are interested in Mantis printhead itself. I have adjusted the wideth of the fanduct so the fan duct does not bomp onto the gantry when X-homing.

It is an about 1 hour print on my Bambulab A1. If you like this one, just feel free to print it out to enjoy!