From the Sumikko-gurashi characters: しろくまのにもつ (Shirokuma's Luggage)
Used to claim corners and when Shirokuma is cold.
Furoshiki (風呂敷) is a type of traditional Japanese wrapping cloth that is often used as a gift wrap, or to transport clothes and other goods. It is not uncommon for some contemporary Japanese people to use furoshiki to wrap up their meal bento box.
Note: The Sumikkogurashi characters inside the sack are not included. They are for illustrative purposes only. These characters are also available for purchase in this website. Thank you.