[HydroJim]

I was thinking pond liner would be make a good membrane. it's durable rubber, but thin enough to make it light.

The only thing I would be worried about is cross winds so maybe the maybe instead of hanging it from cables, it could hang from some sort of hinged arm which would add stiffness from side to side. It would have to be spring loaded then, but it would be worth the extra time to not have the thing blow off in the wind.