There was some research done a while back, by Verus Engineering I think, which determined airflow was much better without the addition of a upper rear window spoiler.
Sadly, all this will be doing is causing drag

If your OK with that and this is purely an aesthetic thing... My guess would be the part isn't sitting flush due to it being warped.
With the part off the car, you could try applying a little heat to soften it up, allowing you to bend it a little before reinstalling (while the part is still warm) and applying a bit of weight while the tape sets and the part cools.

 

Ohhh ok cool, I do have spoiler risers too to go with it. But yeah I’m purely going for aesthetic I might eventually have a front splitter and side skirts to add yet, I’ll have to find something heavy to rest on it while I put new tape on haha, thanks tho

 

No worries.
Let us know how you get on 👍

 

I've heard don't try to stick things like this when its cold. Try heat the glass up a bit and the spoiler/tape. Mines still stuck after several years.

 

Hey pal, car looking very nice. Are those mudflups? Where did you buy them from? Also, where did you get that roof spoiler from?

 

The mud flaps are Rallyarmor,I got mine a few years ago from Tarmac Sportz I think? Easy to put on 😃