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.