Hi & welcome...
Good thing you've already discovered the right foot is the cheapest & quickest means of reducing fuel consumption.
Driving technique will consistently outperform mods, in terms of total potential savings.
As for vehicle mods:
AndrewJ's right: the place to start for max drag reduction is at the back of the car. With the abrupt chop at the end of the roof, and the short deck length, the airflow likely separates at a point that's close to your max projected area, and does not re-attach on the end of the deck.
Not good.
I also agree that rear skirts would help. I've tried skirts on 3 different vehicles, and my conclusion is that the closer the rear wheel is to the back of the car, the more of a benefit the skirts will have. If there's little to no bodywork "aft" of the wheel arch to shepherd flow into a somewhat clean separation, skirts are critical, I'd say.
One other area that you can go hog wild on is your car's underbody. Because of the rear engine layout, you could affix a full, smooth undertray (perhaps uninterrupted, back to the rear wheels) , since you don't have to worry about covering up exhaust components running the length of the car.
And, I trust you've had a look at the
List of aero mods you can do to your vehicle for other ideas?