Someone responded as much in the thread Manfred! A much catchier name than "flag" for sure!
And I got sheets of this material at a Hobby Lobby (Santa's workshop for a studio shooter!) and they are available in different thicknesses. I keep a supply. They have multiple uses and are cheap. Just did a shoot recently where I used a couple of them to clothespin to a strip box to get a shape I wanted. Maybe I should jump back to Donald's studio thread and warn him what to expect about the various odds and ends he is going to end up with!