Cooling Duct that works!

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 postin