I recently purchased a Seasucker mini bomber and a Thule 9003. Since the car is a lease, I couldn't cut the bumper cover for the hitch mount mentioned above. I was able to get two bikes in the back, but didnt appreciate the care necessary to prevent marks on the light interior or the noise inside while driving. Most secure option, of course. See photos and videos below in my iCloud album.
I thought the Seasucker was very stable and very easy to install. Almost no roll or flex in the roof. Takes a minute to wipe the dust off the roof and two minutes to install. Easier if you have a step stool or are tall. I placed the center two suction cups just aft of the windshield, where the top seems the most solid. I had no issues loading two bikes on it and traveling down the highway. Very easy to remove (30 seconds) and stow. My only issue is my wife can't help me load the bikes as it is too high for her. I didn't get the front wheel mounts, so the wheels were inside of the car.
I tried two different mounting methods on the 9003 to avoid damaging the hatch or the spoiler. First was to loop tie down loops through themselves around the hatch hinge. That worked nicely, but the flat hooks on the bike carrier really should be on a rectangular ring. Someone with a sewing machine could easily make some awesome straps with old seat belts. The lower straps could go to holes in the plastic under tray below the bumper cover. I used cloth to protect the paint on the bumper. Seemed plenty secure.
Improved method was I used child safety seat head tethers (about $10 each on eBay) to thread above and below the hatch. The ones I bought on eBay had the rectangular rings, adjustment, soft 2 inch straps and a clip that secured to the hard mounted rings just inside the hatch. I bought 4 in case they weren't long enough. Only needed two! It takes about 3 minutes to mount and ready the straps and another couple of minutes to mount the Thule. Position is plenty high to clear the roadway even when going through dips.
Very solid and doesn't seem to deform the plastic much at all.
As loading the Thule is much easier, I think I'll keep it and resell the SeaSucker.
I have no opinion yet on effect on aerodynamics or noise. Just wanted you to know it's possible and secure.
Short video of SeaSucker
https://youtu.be/HlCwim456Wg
Link to album and videos:
https://www.icloud.com/sharedalbum/#B0nG6XBubgGCWh