Since I have basicly the same car, hope I can share my underbody modifications here. I'm not trying to hijack thread or anything
I raised stock engine cover about 3cm (less frontal area and more air going under the car), removed central portion of stock "airdam", covered whole exhaust and rear axle and created something that resembles a diffuser.
For those wondering what car it is, it's a Skoda Octavia AKA Jetta Station Wagon with slightly different body
EDIT: I attached two pictures with my reply but I can't see them in the post.
Could someone help me with this, please?