I made a taller version (Prototype 2) and made a greater effort at quality; used a template instead of free hand and better cutters. I then went on a 200+km ride managing to actually hit 150km/h at one point (whee...). The results were:
1 - It didn't fly off the bike (I was impressed!).
2 - It made a big difference (I was pleased).
3 - There is a limit to how tall it can be at that angle (I had heard that bigger wasn't necessarily better. Maybe there's something to that...).
4 - The air gap doesn't seem to make a difference at all IMO (maybe something about laminar flow).
As far as I can tell, the quiet spot starts when my eyes are about 2" below the windscreen. On the stock windscreen that is pretty darn low and I would have to get my eyes moved and a new helmet. Adding 3" to the windscreen adds 3" to the quiet spot. But if it gets too tall at that windscreen angle, you start hitting the top edge with your helmet.
Prototype 1 never saw the highway. Prototype 2 measures 6 1/2" extension at the center of the stock screen with a mild tilt up (5 degrees at a guess) to help clear my helmet.
So, the next test is to duplicate the curve of upper lip of the stock windscreen so the helmet can fit into it better. I would expect the extension at the center to be about 4 1/2' - 5". A bit of cleverness can make the lower edge of the extension match the stock screen if it is angled.
That said though, I was doing 110+kph and was able to open my visor 1/2 way behind that thing and there was minimal turbulence and the clear sound of the engine sounding as perfect as an engine can sound.
Today, I took it off to run an errand and felt the exposure to the air again so I think the thing is worthwhile. I agree that the fastening system needs to be better. I'm not too keen on nuts and bolts as they could crack the plastic. I can live with the extension cracking but not the stock screen. I am imagining some sort of plastic rivet like thing but don't know if they exist.
As to the air gap, I am not convinced it is a problem but plan to test with some sort of foam gasket first. Using silicone is a way to go but I would prefer a solution that is removable as much as possible.
But there is a lot of fun in trying to solve a problem nicely and I expect to enjoy it whatever happens; including abandoning it for something better.
It's just an idea after all.