I've always wanted to make custom scarves, armbands, etc. The problem is not insignia or colors (they have custom label programs out), but the media to put them on. Eduard makes prepainted PE, but what can the hobbyist use?

I would recommend heavy, aluminum foil.

For armbands you could use Tamiya masking tape. It's thin, shape conforming. and self-adhesive.

A thin, small sliver of magic-sculp can work well too if you want a slightly thicker look to the scarf or armband.