When I borrowed a friends gopro in the summer I used a tripod mount that looked a lot like a g clamp to attatch it to the roll bar.
Something like
a manfrotto super clamp would be nice and heavy duty and I wouldn't worry about it coming off. Expensive but a very good make in the photography world.
Superclamps on a ferrari
View from the top of the rollbar
gopro by
Series1Landy, on Flickr