When I bought a new hot end from E3D-online I discovered that it wouldn't fit inside the fan duct on my Mendel90, so I redesigned it.
This duct is for a 60 mm fan and has a larger central hole and a slimmer profile, which allows it to sit underneath the nozzle cooling fan.
This version was created in Cubify Invent and is made available as a .FUN file as well as an STL file.
You want the duct to be nice and air tight so don't scrimp on the plastic. You should probably stick to using ABS because of its proximity to the hot end.
I printed mine at 0.3 mm layer height, 0.49 mm width, 50% infill and 100 mm/s speed.
The fan is held in place with four small self-tapping screws.
The duct was attached to the same brackets as I used in the original design (see the 'Derived from' link). These are quite easy to modify in any basic CAD package.