I borrowed a gopro for my upcoming autox this weekend and was looking in to options to mounting it. Looked into buying the suction from gopro for $30, which seems like a lot of people are successfully using externally.
I also found someone connecting theirs to their tow hook. So I bought $2 worth of hardware at the hardware store, in addition to my tow hook and tripod mount. Seems like it may work.
Gopro Tow Hook Mount - Album on Imgur
I haven't roadtested it yet because I'd like to have a safety tether first. I'm also interested in getting it mounted so you can see the steering, so I may end up buying a suction mount anyways.