We had three of the real ones of these in the family, mostly 190 diesels. I've built two of the models and had the same problem with each of them. The rear window just sits in the space and is difficult to fix. Both were oversize and had to be shaved down and then had to stand the car on it's nose and try and use clear fix or superglue to hold it there.
Yours looks superb, just like the real ones that I saw parked nearby when I marshalled at the F1 GP at Silverstone just 5 miles away in the day. Well done.
We done have the real ones anymore as they have changed the latest design to boy-racer hatchbacks that don't have the space we need. We sold them and bought new Renault Capturs.