Sorry I was tired last night and did not want to type a ton.
Here we go so that people don't have to run around and search, I'll provide a very basic resource of an explanation here:
this is a chassis mounted aero component, AKA rear diffuser. Low pressure, high speed air evacuation from under the chassis creates negative pressure under the car at higher speeds. Downforce. The rear underside of the car is rough and choppy slowing the exit of air, causing turbulence and drag. Ultimately upsetting the balance and stability of the car. This piece smooths the airflow and speeds its exit.
This is the often misunderstood component of aero downforce. Usually thats associated with crazy big wings. The underside of the car is equally, if not more, important.
But the Forester is a brick no matter how you look at it lol. Not much will help that...
What angle did you go with from horizontal? Have you attempted to play with the angle, from my understanding as little as +/- 1 degree can make a significant difference, especially when you have an object that is not particularly aerodynamic from the beginning.
Im not sure where I read it, as it was long ago, but there is a set of measurements that you can take to give you an idea of where to start from, and they were fairly easy measurements to take.
Might be worth looking into...
Great job by the way, you can see the labor of love in that