Direct Connection Ducktail spoiler mounting ideas

The problem with magnets is the surface area. You have a fairly large surface area for aerodynamic forces, and a pretty small surface area for the magnets.

You’d be better off with something like 3m adhesive tape. The psi for the bond is lower, but the surface area is higher. 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.