I have been running an E3DV6 style hot end for a very long time and have had issues with the fan ducts for the 30mm square cooling fan, as have had many others.
I have had several ducts fracture like the one shown in the photos, and have split out the corner holes on a couple as well. They seemed to be a bit weak to me and I suspect that the plastic is hardening with age and repeated heating. At the same time I also had problems with the fans themselves and decided that they needed to be tilted slightly to better run against their internal bearings.
I looked around the site but preferred to design my own version. So I have designed and tested two versions from scratch that address the problems and are reasonable sizes. They have more generous plenum spaces behind the fan and are quite strong while not being too difficult to press on and off the cooler fins.
*** The first version is basically a re-creation of the stock duct but with a wedge and a small plenum space. The second version is identical except that it has a 1mm flange added at the fan mating surface in case you may have any difficulties printing the very thin walls at the quadrants of the fan opening that the first one has. Both print nicely and work well. ***
In the process I also decided to create and add a small fan grille to protect it. I chose to make it domed for strength and cosmetics. I have attached two versions, one of which has an internal dome as well to slightly increase air flow.
So now instead of having to order replacement parts as I did in the past, I can simply print new ones. But so far they have held up very well.