I printed a selection of fan ducts for my Mendel90. Some of them didn't fit and others just didn't work, so I designed my own. I have copied some of the best features from other designs and merged them into a model that will fit on the Mendel90 using a couple of adapters for mounting.
Update - April 2013 - my original fan duct was getting a little overheated so I finally got around to redesigning a slightly longer version which centres the nozzle better.
Update 2 - the all-metal E3D hot end needs a cooling fan and shroud fitted to it. There was not enough space to fit this so I have released an updated low-profile version at thingiverse.com/thing:91931