Update: V2 for Cura users-
Since you can't manually place the supports, this version has built-in support struts inside the neck of the fan duct. So the entire thing can be printed without supports.
You can easily snap the struts off with a pliers after printing.
This is a remix of juha_j's fan duct (which uses the Noctua 40x40x10mm silent fan NF-A4x10 FLX), with the following changes that I made:
1) thickened the duct walls slightly for improved rigidity.
2) slanted the "neck" of the fan holder to get a more streamlined airflow, as suggested in cannon3d's comment.
3) lengthened the duct and added a fixing collar at the end. This way the hot air gets expelled directly out through the ventilation holes of the metal base.
I've included the .skp files (in Sketchup 2018) so feel free to make improvements if you wish.
AnyCubic i3 Mega Ultrabase
Can be printed in PLA or preferably PETG (I had too much warping with ABS).
Print the models as oriented in the Thingiview images.
This way you'll only need a short row of internal support at the neck of the fan duct, which can be easily removed post-print (for Simplify3D users).
For Cura users, print the V2 file in the same orientation without supports.