So, I re-did the mod. This is the way to do it, imho. Way easier.
Just fab a piece of 0.060" AL with a 2" hole. Cut to fit plastic NACA duct and Van's supplied AL bracket. Then match drill the three pieces and install screws (which can be machine CS if the flat AL you fab is 0.060" or thicker).
This is better than my first prototype because now the eyeball can be removed if needed later. Like when a bird gets caught in there.
BTW: That larger Steinair vent fits the Van's whole better (it's 2.75" vs the 2.6" hole provided), but it's almost 4" deep, which makes it necessary to modify the plastic NACA duct, as the previous poster indicated. I agree, this is a much more difficult way to go.
I also agree that doing an extra 12 hours work just to get cool air ducts might not be for everyone, but it's these little custom touches I like and which add to the fun of building a one-off flyer. And, these vents are really nice pieces. Very high quality workmanship.