When I received my Ultimaker, the one piece that was faulty was the 50mm fan used to cool the extruder. I happened to grab a 40mm fan and needed a duct to go with it.
Modified Berend's Ultimaker Ducted Cooling Fan such that the really long screws on the extruder head can screw directly into the duct's roof (as opposed to replacing the screws with even longer screws as in Berend's design).
It was specifically designed to be printed without a fan and, thus, is dead simple in design.
It can be held together with hot glue.
STL files attached. It should work on any extruder configured like mine (extruder at front left with cables sticking out back left) or any rotation therein. If your extruder is built differently, you can likely mirror the STL appropriately.
Mount the fan to the roof using at least two screws.
Print out the two pieces and either glue them together or use some wire ties as I did.
Unscrew the long screws on three corners of your extruder and screw them directly into the three mounting holes on top of the ducting. You'll need to press the duct onto the screw pretty solidly when tightening it down. It helps to hold the duct next to the screw prior to unscrewing it to know how far you need to unscrew to have enough threads to screw it down.
When bolting it in place, make sure that the air exit ports are surrounding the nozzle and make sure that every single bit of the duct work, including whatever you used to hold the top and bottom together, is higher than the extruder nozzle!!