Even with a proper tow bar, the rear weight bias of 911 is going to make stopping dicey. If you brake hard the rear end is going to try and out run the front, crumpling the bar.
Ive towed with a tow bar before, just remember that the "trailer" has no brakes, and the tow bar is nowhere near as strong as a real trailer frame is.
Brake like your driving in ice and you will be fine, otherwise you can end up crumpling the towbar. Most of the RV's that trailer cars like this have systems that use the cars brake as a trailer brake or use far more heavy tow bars.
I was using my old truck to tow a camaro, where a kid ran out in front of me on a 4 lane low traffic street. I was doing 35 mph, and had to brake hard, managed to stop safely, but the to bar was crumpled on one side, bent on the other, the truck and car tried to jacknife together, damaging the trucks bumper some but doing major damage to the car.