Meanwhile I've tested a royal blue ribbon, held against the light the blue looks almost fluorescent (hard to capture with a camera).
The material weighs about 223g/m2, so, a band of 1.8m x 30m would be about 10-12kg. Added to this will be the pull needed to keep the bands straight. Made of polyester this ribbon is strong and has little elasticity. Yet when I spanned a length of six metres I found that it was quite flexible which is good. Little effort was needed to straighten it.
I made a small sample-band - not to scale - composed of 6mm ribbons. Each band is twisted by 180 degrees, as if going with the movement of the sun. So the light is caught differently - which would show better using a wider ribbon.
This trial seduced me to order a bit more Grosgrain - enough to make some bigger samples.