Her hands on your hips is far better than her holding onto a piece of metal behind her back.
I know that you said no top box but Peters suggestion is probably the best one and all you do is for you and your wife to learn to get on the bike a different way. Have the bike on the stand. She steps up onto the left rider peg gets onto the seat and slides back (like getting onto a horse). You then face the bike and kick your right leg up onto the seat, slide it over and get on. It's hard to describe but with some practice it works and plenty of riders and short pillions do it.