Sam69,
This is something I've been kicking around as well... based on what I can figure... the Voltex stuff works. So, one option is to pay The Man.
Here is something else that I may try. It may or may not work. It's cheap though, and wouldn't require much financial commitment. It essentially tries to copy what Blackbird Fabworx is doing on Miatas. I've owned one of the Blackbird Fabworx lexan spoilers for a Miata. Never installed it, sold the car, gave it to a buddy. It's on his supercharged Miata now, and it's "doing something"... we aren't quite sure what yet. That car is a bit of a handful.
Here is the Blackbird piece:
Anyway, I plan to get something like this:
I have some lexan in the garage that I can add to it, to increase the height of the spoiler into the air, but not hurt rearward visibility. I'll need to attach it with fasteners, heat with a heat gun it to mold it to the contour of the spoiler, trim it, etc. I made need to support it with struts and I may have to drill into the trunk. Fortunately, I have access to a spare trunk in case I need to do that. The only time this car sees the street is if I decide to drive it to the track rather than tow it. Not sure I'd try something like this for a street car (unless I had a spare trunk to swap for an event).
I have no idea how well it will work or whether I'll do it, but it's cheap enough to try.