This is a nozzle fan duct for 50 mm radial fan. Its mounted on my previous bowden extruder mount. https://www.thingiverse.com/thing:2766948
I have tried circular ducts, double-sided ducts but I believe that this is the most efective duct. You can connect the fan power cables to the connection D9 (12 Volts) of the ramps board. So the nozzle fan is controlled by the software.