Mr. Julian Edgar!
OMG! You are here! I have been following your work on Autospeed.com for years! You are inspiration sir! You and your team's work is nothing short of brilliant. I am totally speechless right now.
Sorry, as a contribution to this thread I would like to provide an overview of an air deflector I had made several years ago. I had a blog about it you can see it here
https://neoinfoxchange.wordpress.com.../aerodynamics/
Initially it was an excercise of a modulated spoiler on a hatchback that I had linked to an arduino. Then during the tests I found that I could reduce fuel consumption with a deflector rather than a spoiler. My hypothesis is that air over the edge of the roof can be directed towards the rear and reduce the size of the trailing wake.
The blog post includes a screen grab of a video I had put up at Flow Illustrator; an online 2D flow visualization software.
Before Deflector
After Deflector
The idea was that this could be made commercially available and would be non intrusive to existing body design. Allowing easier adoption to the masses rather than a full blown teardop mod.