The Gurney lip(s) would probably be declared illegal as they would provide more downforce
More seriously, a pair of flaps, flush mounted, would work well, I believe. Flaps are required in the cowl area of NASCAR Cup cars; you can see them bopping around when drafting is close on superspeedways. We used to hold them down a bit tighter with soft springs, but that was banned.
A few years ago we took a record-holding (over 200 mph) classic coupe to the A2 wind tunnel for evaluation and recommendation of aero improvements. The car had an Implementation Approach 1 spoiler. As part of our quest for knowledge, we made our last run with the car BACKWARDS (180*). The result was approximately 750# of lift on the rear of the car. (If we had been at Lockheed, we could have yawed the car in a controlled 360* spin, measuring aero change every 5 or ten degrees of rotation. THAT would be interesting). Needless to say, we recommended flaps or hinging as part of the aero improvements. My thinking, it can't hurt and is pretty simple to do. Also, a hinge need, follow the body contour, just place the hinge at the most readward point of the contour. And tethers are simple to make or source.
John, think of the flaps as burst panels for your car.
vic