I really wanted one of Iwo's ducted cooling fan ( http://www.thingiverse.com/thing:5756 ), but only had a 40mm fan, so I printed his thing, then made an adapter ring to fit my junk box 40mm fan.
Print Iwo's ducted cooling fan http://www.thingiverse.com/thing:5756 then print one of these adaptor plates. It just presses into the top of Iwo's design. You could drop a little acetone onto the join to glue it, but I haven't found the need. I have found a little strip of double sided tape is necessary to keep the whole duct in place on the build platform (can be seen in the photos through the bottom).