Bill,
I have a couple of suggestions for your design. Your string looks a little short. I think it is possible to use a 25 foot string for long distance as long as proper tension is applied and maintained at all times. I once saw a version that used wax coated paper cups. The string was then connected internally using shirt buttons so the string would not pull through. I think the added flexibility provided less signal attenuation. I don't know if those kind of cups are available anymore but perhaps those plastic Solo cups can be used.
Dale