The website mentions silk organza at https://1worldflag.com/flag/. That's where my mind went initially, some kind of sheer silk fabric. I don't know how durable it would be in the elements though.
What a great way to unite the world in a practice of cruelty to its inhabitants
- 1worldforhumansonlyflag - 1worldindifferenttosufferingflag
The whole idea falls flat on its face because of this for me.