I have updated this yet again and it now fits the Printrbot aluminium extruder (or the plastic version http://www.thingiverse.com/thing:281015).
It needs an adjustment for the holes for mounting the blower, but I've now just stuck the blower on with double sided tape. I didn't complete it because I've now moved to the new Printrbot carriage design.
You'll need to print this in ABS, then acetone glue on the little air dam on the side.
It's designed to take a 50mm 12v blower on the side. The reason I opted for this is because the efficiency is better and the design is dramatically cleaner. It also made some room to fit an autolevel probe. Also the blower is a lot quieter than an axial fan.
Anyway, more than anything I've put this up to give some ideas to modders, and having a mostly complete design to work from saves you a lot of time in the drawing seat.
I've probably done 1000 hours printing on this carriage and the performance has been brilliant.
Edit 1 - Forgot to include autolevel probe