When I had the suspension done on my weekly driver we lowered it until it looked *good* and then raised it about an inch so it only scrapes on some of the speed bumps. Apparently it helps to traverse them an an angle, if possible.
OTOH, my VW panel van can roll across the extruded curbs that they use in parking lots. It would get spats before lowering.
The spats are on the aero template are optimal, the factory mud guards are the other extreme, and anything in between would be beneficial. A reasonable material would be conveyor belting. It's made to take abuse and would just flex and rebound. Here's a sketch done for a pickup truck:
You might also look at the approach on Basjoos' Civic, a double walled air dam between the front and back wheels. It seems to work for him.
To get the buttresses to work with your soft top they would have to be inflatable. And to reach the template line they would have to be like tail fins. And then they would only affect 20% or less of the air moving over the top [maybe].