Hello jedi_sol and welcome on-board.
Rear wheel skirts are easy to do with cardboard or coroplast.
You didn't talked about your driving style. Since I pulse & glide (in neutral, engine on) I improved a lot: the secret is to wait, wait, wait...
I installed coroplast rear wheel skirts at the beginning of the current tank and I can feel the car coasting longer. I also feel it more nervous and my "fast accelerations" habits are coming back
but I will beat my records
I'm dreaming about a kammback, but I'm afraid tape will not be enough to keep it securely attached.
Have fun,
Denis.