Check out this guys solution. He says he uses a no ground plane antenna for vhf/uhf ham. Found one similar here although it says it's a tunable CB antenna from Firestik.
Then I saw this, it looks like maybe a glass mount or lip, I don't know. He's going to fast around Nurburgring to see it very well
I don't know if any of these are options you would consider but it was fun looking at what people have done. Good luck.