I did a simple but (I think) useful update recently :

One detail that was bothering me is that the diameter of the funnel base was less than the fan's aperture (you can see that if you zoom on gallery pic #6).

So potentially a fraction of the airflow would be lost outside the funnel.

Also, the plastic-on-plastic contact of the funnel base with the fan top was a bit slippery.

While puttering around in my workshop I came across this sizeable length of rubber insulation :

As you can see it is has two parallel tubes, and under each one is a sticky track (under the yellow cover. So I cut a length of the funnel's outside circumference, and stuck one tube outside and the other underneath / inside :

This way all of the fan airflow goes into the funnel, and the coupling between the two is really snug :-)