Not upside down. There is a slight curve in the edge of the trunk lid and the spoiler is shaped for that on the rear edge. The front edge is straight.
On the magnets, I would use the trunk skin you see in the pics to use as a metal base . That would then mount to my painted trunk with magnets imbedded in the spoiler base (donor trunk lid). The skin will, now spoiler base, would be perfectly contoured for a good double sided adhesive too.