Hi there, I love your Totoro costume and I think I may have come up with a rather simple system for a waving arm mechanism, if you're still looking for one. The following are two pictures to illustrate what I had in mind. The frame itself would be attached to the backpack, and the position it's shown in here would be at full tension, with the arm in the upright position. With no tension on the line, the arm would simply hang at your side like normal. The frame could be made of lightweight wood and/or pvc tubing so it wouldn't put stress on the costume. I'm a bit tired at the moment, so I may not be all too clear, but if you have any questions, I'd be more than happy to help in any way I can. Hope I can help, and again, I love the walk-around Totoro!