This is remix of llapao's "Anycubic I3 Mega 5015 duct fan cooler ultrabase".
I chose the crescent-shaped duct version as it's the most compact design that doesn't obstruct your view & access to the nozzle.
So I flattened the crescent duct further to make it work with the E3D v6 hotend, which I had replaced my i3 Mega's standard hotend with.
The duct fits the "5015" 50mm x 15mm 12V blower fan (I'm using the "SoundOriginal" brand from Amazon, that can run in variable speed).
For best cooling performance, update your firmware with 3D_Rapid_de's 12V Fan0 mod.
Anycubic i3 Mega Ultrabase
Best printed in PETG (or ABS if you can manage the warping) since the duct is so close to the heater block.
The duct is designed to be printed flat on the heated bed (see the images), without requiring any supports.
You may need a brim if the first few layers have problems sticking to the bed (especially the crescent tips).
Fix to the X-carriage with M3 x 8mm screws and nuts.
You should get a 2.5~3mm spacing between the bottom of the duct and the nozzle tip.