Fill the thermos with boiling water, close the lid, and let it sit for 5 minutes. This warms the inside so the container doesn't pull heat away from the soup.
Heat the soup
While the thermos warms, reheat the leftover potato soup on the stove or in the microwave until piping hot.
Fill the thermos
Carefully empty the hot water and immediately add the hot soup. Close the lid tightly and pack it upright in the lunch bag.
Pack the toppings
Pack bacon bits and shredded cheese in small lidded containers.