NOTE: use the v1.1 version (or later if available). I found some of the driver zones didn't have the expected air flow so I revised several things. Please be sure to test the air flow using a thin strip of paper on the exhaust ports. I revised the model but not the photos, so don't be surprised if it looks different.
I made cooling ducts for the DuetWiFi, DuetEthernet, Duex5, & Duex2. The motor drivers on the Duet 2 series boards, TMC2660 are best cooled from the reverse side. Machines using currents in excess of 2A should have forced air cooling, the Duet3D project recommend reverse side cooling. I didn't see any ducts that I liked so I made my own. The channels were designed to redirect flow as simply as possible, and they cool the reverse side of FETs, regulators and the microcontroller. Another bonus of reverse side cooling is that it leaves the wiring & connection side easily accessible.
I don't want to duplicate other people's work so if you want board CAD files go here:
I designed this for the Delta BFB04512MD blower fans. It's a 45x20 blower, with a 20x27mm exhaust. This model has intakes on both sides of the fan so that's why there's a hole in the fan bracket. This duct should work fine with other 40x20 and 45x20 blowers with a large exhaust exit. The part is intended to be drilled for fan mounting screws. Drilling is easier than altering the part that shifts due to warping or shrinkage.
I've also included STEP and Alibre CAD files so you can adapt the design for your uses with your favorite solid modeling CAD software.
Any FFF printer should work. I designed the walls to be 1mm thick so I suggest setting your slicer to 0.5mm extrusion width. I didn't need any supports for these parts.
The parts are "cut" in half for my Ultimaker 2 Go (my backup printer) so a printer as small as 120mm on a side can still print it.