Photocopy off the back of the how to paint space marine book
Sicker banner from the command squad box:
Painted plastic (from the command squad box):
If you have the greenstuff skills to make a smooth sheet, that’s probably going to be the most natual looking. You might be able to get similar results with foil, or heat and bend thin plastic.
Best of luck, not a big fan of trying to make banners work myself.