When I look at the view from behind I get the sense that where the top and sides meet at the back of the extension,that it could be narrower,and I'm basing that on what I can see of the 'tumblehome' of the Matrix.It's amount that the sides of the car tumble in at the top toward the center of the car.
A view from the top would be helpful but I realize it was late and had run out of natural light.By the way,I like your light stand!
If the transition onto the extension is sharp,as Darin has mentioned,you may see some separation although I don't think the air would travel far before re-attachment.
Compound curves rule the roost but are also the most energy intensive to fabricate.
I can't help with coroplast techniques,I haven't worked with it yet so I'm useless to you there.
