My previous shot at this didn't manage to blow too much air onto the nozzle. Some people were even beginning to try dangerous things to get it to blow harder, like blocking the air path through the radiator fins.
A better solution is, of course, a beefier fan! The standard Mk7 fan is rather wimpy, but you can get 4mm fans which will blow much harder (see the second picture for a comparison). Since there's more flow all around, we can even divert a larger portion of it towards the nozzle, so I changed the duct to do just that.
With this setup, I get quite a strong air flow around the nozzle.
1) (optional) Use a 3mm tap to thread the top holes
2) disassemble the Mk7 extruder
3) re-assemble using this duct instead of the fan, using the long bolts that hold the original fan in the duct's bottom holes.
4) fasten the fan to the duct using two M3x16 bolts through the fan's top holes.