A 1/4 - 20 bolt, nut, two washers, and a lockwasher is best, use locktite to keep the bolt from coming undone. The length should be enough to allow all threads to pass completely through the nut. You may have to drill a hole through her head to get to the bolt but you can use an allen cap screw for that instead of a hex-head bolt. That way you will have a smaller hole. Then, if she falls out, you KNOW she is undoing the bolt herself and just does not want to fly. In that case, follow the above and find another pilot.[X(]