If you're going to consider fiber you should really look into cotton twill. It can be run over a long distance and is reasonably dimensionally stable, maintaining its sharply defined bandwidth over a wide range of distance. It must be maintained at constant tension, of course, and if you want it to be longer, you can't actually stretch it at all. It won't stretch, and if it would, you'd have standing waves.

The one drawback of this cotton fiber is its susceptibility to crosstalk, literally air movement across the device causing a modification of the desired signal. Crosstalk in the original sense.

The transmitting and receiving cans utilize standard industrial sizes.