If you want efficiency I would recommend looking answers from efficiency cars. In race cars you cannot know what is there for downforce and what is to lower drag.
But what you can know is that MB EQXX with Cd 0,17 has those rear vents. They help I have also some studies on my computer. If you plan to build ones look for a car that has similar width tires etc and copy first dimensions from that.
In my Audi A2 bodykit there will come those vents as well. 3d printed parts are easy and fast to try.