Don't be afraid to use a spook to freehand the design. I tested bags and squirt bottles, but they made a huge mess and were difficult to use because soup is hot. The spoon method is easy to use. Like Julia Child always said, "You must be brave in the kitchen."
You can also serve this in a cool reverse effect, using Black Bean Soup as the base and the Butternut Squash Soup as the design. This also helps the soup stretch further because more soup is needed for the base than the design.