Flying and claustrophobia...?

My wife can fly in large aircraft with no problem. I have taken her up in my small plane on nice, calm days and she is okay for a little while, but then she gets nervous and sick. I don't think it's motion sickness. She does not fear flying, but she says that she's antsy about being in such a small, closed in pit. What can I do to help her with this? Is hypnosis a viable option? She would love to share the experience of flying with me, but she has tried several times with the same result.