1. On the two "front" studs (the ones closest to the third brake light), find two equal height nuts that are the same thread pitch as the studs and original cap nuts. Put one on each stud and tighten down as much as you can.
2. On those same two studs, install the original cap nuts (tighten as much as possible again).
3. On the hatch, use some sort of water-tight removable material to cover the holes the two studs in question originally installed in. (I find two strips of electrical tape works perfectly).
4. Reinstall the spoiler in reverse, except only bolt in the "rear" studs. The posts you made out of the original cap nuts and extra spacer will rest on the taped holes.
I had this setup for YEARS. No damage to the car, no damage to the wing, and no water leaks.