The "ears" on the DKW posted by Aerohead looks terribly silly. I guess they can improve stability in high speed but they must also incrase sidewind sensitivity. Saab may have been designed by aircraft people, but I have no intention to add rudders and fins.
Except for that, the DKW attachment is quite close to what I plan as a first step. if I remove the rear bumper I'll get two strong lower fixing points and I also plan to install a hitchball. By using a trunk hatch as the base for my kammback extension and combining this with the lower fixing points I'll hopefully get the whole thing stable enough to not need any higher fixing points at all. The small gap between the kb-extension and the car's body must be filled with a rubber seal or some long lasting duct tape, or my dogs will be soaking wet when it's raining...