Addon piece for a Printrbot (www.thingiverse.com/thing:16990) that sits between the extruder and extruder mount to attach a 40mm fan. It should work with the printed or laser cut versions. I don't have mine assembled yet, but others have said it works. I'll probably revise the mount once I get my bot printing and can see what can be improved.
Print the mount, put it between your existing extruder and extruder mount. There are clearance holes for the screws and hot end to go through. Tighten everything back down and attach a fan to the mount.