I'm not sure how important it is to close the gap between roof and start of spoiler.
In studying the template,
( see these links:
http://ecomodder.com/forum/showthrea...tml#post266562
and
http://ecomodder.com/forum/showthrea...tml#post116820 )
I found that ideally, the roof extension for this generation Civic coupe needs to begin several inches forward of the rear window's top edge. So I wanted a thin material that could hug the roofline.
If I had fiberglassing skills I could build this project that way. I just happen to be partial to 2.7 mm. lauan plywood, primed with oil base primer. I thin the paint a bit so it's not too goopy.
The first link above was my attempt at a cooler looking design. However it wasn't very successful when I did coastdown tests. And the panels wanted to move a lot under the pressure of air flow. The design in this thread is stable at highway speeds.