Update 23-Aug-2018: I've published an updated version, but am leaving this up as some people quite like the nozzle viewing cutout. New design is here - https://www.thingiverse.com/thing:3063554
This is an alternative part cooling duct for use on an Original Prusa i3 MK3. Print, swap out the old one, carry on. Exits are aimed just below the nozzle and from 3 sides.
I've also added in a viewing slot so that you can see the nozzle a little bit better (similar, but not as open as the one from here https://www.thingiverse.com/thing:2065461).
I'll add some more photos of the final part on it's own once I've printed off this iteration, which only has some text and cosmetic fillets added.EDIT: Added photos
I printed this part in PETG (specifically Extrudr's MF-PETG in Neon Orange) and it holds up well for PLA print and short PETG runs. Print in ABS or better if you plan on using hotter materials.
Designed in Rhino, with some fillets and the text added using Fusion 360. This is iteration 14, so it has been tested quite a bit!