I used to have this cooling duct on my i3 which inspired me to design my own magnetic delta effector with a built in print cooling fan. I also wanted to put a capacitive probe on it, which it ended up in a pretty awkward position right in front of the E3D cooling fan, but it doesn't hinder airflow that much. All in all, I'm extremely pleased with the end result.
There are no nuts for the magnetic balls, I just screwed them right into the plastic and they sit pretty tight. Also be mindful that the arms won't hit the fan or probe when near the edge of the print area, but it should be ok. And lastly there doesn't seem to be any issue with the plastic (PLA) closely surrounding the heater block. I've been printing with it for many hours already.
There are a few versions, most have a spacing of 48 mm between the rods, one with and one without a probe. There is also one version which has a spacing of 51 mm. Step file now included as well.
Edit 26-02-2017: By request, I've added a version for KARM-06 CL Igusball bearings.
Edit 16-12-2017: Someone reported that the fan is also blowing air on the heatbrake and heaterblock. To eliminate this issue I have uploaded revision 1. I have not tested this but it should still fit just fine. If anyone tries this new revision, let me know what your thoughts are.