This is what I would do (only because I love borders and 18 inches is a lot of space to cover).
First, 1 or 2 skinnier borders to pen in those flying geese. I like the darker colors.
Then a border of the cream, about the withe of the first 2 combined, or double the one (if you only use one)
Next, some more of those flying geese because they are beautiful (You did a beautiful job) and it would bring an element from inside the quilt to the borders (which is something I like to do).
Then repeat the cream border.
Then repeat the first 1 or 2 borders.