Ok, time for some creative design work.
I just installed this mailbox and now I need a flag for it.
My mail carrier (who says she really likes the new mailbox) said that if I just put a clothes pin on it the lid, she'll know that there's outgoing mail. I'd rather have something a bit nicer than a clothes pin. Over the past few years we've changed the door handles, light fixtures, house numbers and now the mailbox to brushed nickel/stainless steel. The mailbox is mounted on a white wall.