lol but I can't draw freehand, so any cad software that relies on you drawing stuff, leaves me cold. Plus I haven't found anything I can't make with openscad yet.

Where do you envisage putting your fan ? You duct has no input point. If you lay the fan flat, most of it will be blocked by the carriage.

Currently thinking about a clip built into the top of the carriage, so the fan can just clip down, standing vertically and the duct can clip onto the front of the fan. I can also stick something in the gap to keep the duct fairly stable.
I'm thinking more about a straight shaft with an angled exit, rather than a right angle. You need to blow down towards the bed and area being printed, not just across at the tip. You also don't want the duct on the same level as the nozzle as that could interfere with prints. So it has to stop above the nozzle and blow down and slightly across.

It does help having the printer in front of you :-)

Oh yeah, even the pla carriage I knocked out last night at 0.3 layer height, 75mm/s with no cooling fan - is way better than the one it came with.