Why not use double sided 3m tape for automotive? This is how my hood protector is affixed to my Bronco and it's solid as a rock. Is there a flat edge on the underside of the spoiler where it mates to the deck lid?
No, the ones from Showcars on EBay are just the top shell. No base. It’s very very basic. You’re on your own to come up with mounting.
I’m hesitant to poke holes in my trunk lid so I would like to make a base for the top shell to affix to and then mount the thing , as a whole unit, with magnets or 3M tape.