Direct Connection Ducktail spoiler mounting ideas
You could build it up the backside of the spoiler to have a flat surface to mount the tape to so there’s good adhesion.
Before I proceed with any cutting or drilling I am going to make a couple of *plugs*? IDK what it's called exactly but basically lay some fiberglass shapes/profiles of the deck lid. One can be bonded to the underside to be used w/o the donor deck lid/metal base idea. That way I would use 3M double-sided adhesive tape, or like, and just make the fasteners ornamental.
With the metal base idea I would be using fasteners
McMaster-Carr
I would also cut the trunk skin and use short fasteners to attach the spoiler for a rigid base, then fiberglass a base plug to cover the metal. It would be a flat, matching profile with the painted deck lid for a superior bond.
The magnet strength isn't a concern for me. I am confident that with the arrangement I have pictured (2 round, 2 bars) would hold. The concern would be magnetizing the trunk. FOr this I would use a type of Ultra High Molecular weight hydrophobic sheet for the painted. This would reduce fastener to paint interference.
I wish I had two spoilers and two donor deck lids. I would do a competition to see which one to use and also have spare.