Can you bend the mirror arm forward enough so the door doesn't hit the mirror? A direct fix might be more appropriate than adding something else that might have it's own problems.
A nylon strap that's coated with vinyl would be strong enough, and might match the seats, but given the "older" look of leather, it might be more appropriate. That or seat belt material which is available in multiple colors. Or just nylon strap in the appropriate color. Check
HERE. Available in a wide range of colors.
