[razordave]

My roof extension really didn't turn out pretty at all. It seems the top side would need to have a flat general shape with a bubble in the center to follow what vw left.

I'm going to leave it for at least 100 miles, it's already there and there's something going on back there based on how much dirt is collecting.

If there isn't a significant improvement, I would have to say I'm going to do my best to replicate Ernie Roger's spoiler.

I have a pile of holes in the roof just ahead of the hatch where the rivets currently attach the extension.
That may be an invitation to add a fixed spoiler along that edge. Maybe something similar to the factory active spoiler on the turbo s (attached image)