The problem you’ll have is surface area, as you know that spoiler is totally hollow and the only parts that sit flush on the trunk are the screw pockets.
So, what I would do is get a couple of thin strips of aluminum. Thin enough that it’s easy to bend but thick enough that it’ll capture some threads. Use the screw pockets on the spoiler and screw the thin strip of aluminum to the backside of the spoiler, then flush the protruding screw points so the part that goes ro the car is flat. You’ll probably have to use 3 or 4 pieces so you can leave out the part that goes over the rib in the trunk.
Then use a 3m body tape between the aluminum strips and the trunk lid.
