Just for your information, a 3’ by 5’ standard flag is .5 mm by .8 mm, or around the width of a mechanical pencil lead. Even a 10’ by 15’ flag would be 1.6 mm by 2.4 mm.
I recall seeing somewhere that simply printing the scale of flag and cutting it out, then gluing to a bit of tin foil was a technical someone used. Of course I cannot find the thread right now, but it does allow you to bend and get that sea-swept wind look. As for gluing it to the masts I think it’s thick enough to stay put with a bit of clear cement glue. I’ll see if I can find the thread of it...