Cob cob balance beams

[Unleesh at aol.com]

I work as a therapist and I want to construct a balance beam in the back of my
studio. I intend to have clients walk across it while talking to me about
their situation, and note when and where they start to lose balance, as an
unconscious indication of where they are "off balance".

I'd rather not use wood.

Any recommendations for making a cob balance beam, say a half foot to foot
tall, 15-20 feet long, 3-4 inches across?

Is there a way of waterproofing it so the rain won't simply wear it down? What
if I put plaster or something over it?