This is an improved variation of philsson's 40 mm fan duct. http://www.thingiverse.com/thing:184055
There is now a bit more height at the two screw mounting slots.
I improved the air flow in the duct again, by creating a transition between the main plenum and the ring duct. There should be even more air flowing to cool the extruded material now.
I've also directed the flow of air around the nozzle a bit better. I had printed a few big herringbone gears for a Wade style extruder and they came out distorted. Turns out that most of the air flow that is directed at the extruded plastic was coming from one side. I've blocked some of the air flow in an attemped distribute it more evenly. It seems to work, the distortion problem stopped. The latest revision balances the exhaust to match the clockwise spin of most 40 mm cooling fans.
I've tweeked the now 8 mm by 15 mm opening to help provide cooling air flow to the PEEK body of the J-Head. The theory is that it would reduce the chances of PLA jamming. It may also be beneficial for use with ABS, but I'm not using it with that kind of plastic at the moment. On the down side it might cool extruded ABS too much.