Thanks for remembering my old box cavity study.
The results were totally equivocal, and the experience put me off on the idea that there was anything much to be gained. However, I have wondered a little if it wasn't because the box cavity was a bit on the small side. Since then I have added a rear diffuser, substantial wheel well covers, & smoother, closed wheel rim surfaces. Studies of these kinds of mods that I have seen tend to show that there's a lot of interaction between the different mods. So, I have wondered whether a differently shaped and longer trunk lid spoiler would have a better effect. I don't know, but I think there's reason to think it might. And I'm going up to El Mirage this weekend anyway...
On the basis of photos of dust and water trails on the car, aerohead taught me to see the vortices coming off the C Pillars and spinning off behind the car:
The tufts and the dry zones in the mist water suggest both an area of recirculation near the middle of the rear window/trunk and vortices spinning I guess clockwise off the c-pillars as air from the top collides with air from the side. So I am thinking of a shape that might reduce the energy in that vortex. I think I might try a design that wraps around the curves at the corners over the tail lights. I will make it a little longer than legal, too, probably, given what you guys have been running. If I understand correctly, there might be potential there. I am probably wrong again. lol