PLEASE do us all a favour and don't even think about those silly 3M tape!
The other drivers won't love to have your spoiler sticking through their wind shield
A spoiler is an aerodynamical part of your car where enormous forces will appear!
Please use the stuff (black glue, like sikaflex) that's used for replacing the wind shields.
Sorry, I don't know the exact term....
There is also a glue sold by BMW for the spoilers. (same thing)