Looks like everyone uses more or less similar setup. I was bought cheap $15 suction cup from eBay, but it's quality was quite low and it just fell of even from glass window. Highly recommend original GoPro suction cup.
As a safety measure I also got cable with carabiner attached, but after riding more than 2000 km suction cup never fell off.
Also, if you feel uncomfortable riding with expensive camera attached outside your car, get Xiaomi Yi - it's image quality is quite close to GoPro and it costs $70 and can be attached with all standard GoPro mount