1) the front windshield and middle where the front and back doors meet, or
2) the middle where the doors meet and the back window on the hatch. Those match the places that feel relatively solid when I press lightly on the roof too. I can't tell if there's enough solid width in the middle for the double 6" SeaSucker cups.
Because of the curvature of the roof, it looks like it'd be angled very "downward" towards the nose with option 1 (front). Option 2 looks like it'd be angled quite upwards. I'm not sure if that would be a concern for the amount of lift depending on cargo. Hmm...
Another potential option I found is a Rhino Rack with clamp feet. But their site doesn't have a specific recommendation for clamps (they only talk about the fixed point mount for pano roofs for Model S) and looking at how the trim is done around the door, I'm not confident of them being able to solidly attach, preferably without crushing or scratching something.