This is a modification to the original E3D-Online v6 hotend fan duct. It mounts exactly the same way as the original. This duct extends just a bit farther down to accommodate a 40 mm (4 cm ) case fan. It also extends outward and additional 5 mm to clear any obstacles and minimize back pressure.
Get rid of the small 30 mm fan and put something with a little more power and life in its place. It is not designed to center the fan. It is placed on the heatsink fins exactly in the same position as the OEM fan duct.
Print using Nylon, PET or ABS. Nylon and PET will be less brittle than ABS and more flexible for snapping in place over the heat sink fins. Print face down with a brim to prevent warping, also allow your slicer of choice to add supports to the underside.