Judging by the many postings of late, lots of folks are trying to make a cooling fan shroud or duct that will properly cool their prints, but aren't have a lot of success. I believe the reason for this is that all the designs are using axial fans. Axial fans are designed to produce a lot of flow at low back pressure. As soon as you add ducting, the back pressure reduces the flow rate rapidly and you get little, if any, cooling.
The right answer is to use a centrifugal fan. The duct I'm posting uses one, and believe me, it provides plenty of cooling air for your prints.
UPDATE: The FreeCAD files are now included in the archive.
The fan itself is readily available on EBay for around $5US or so. Look for DC-2pin-12V-50mm-x-15mm-PC-Cooler-Cooling-Brushless-Blower-Fan-Hole-to-Hole-57mm. Look at the pictures to see how to attach it to the duct. You'll probably have to make your own mount to your X carriage. I provide an example and various length links to mount it. Again, look at the pictures.
Design files in FreeCAD are available. I will post them if there is interest.