The ornamental design for a flexible ball bag, as shown and described.
FIG. 1 is a top, front and left perspective view of the flexible ball bag showing a carrying handle oriented substantially perpendicular to a zipper and having at least one open mesh fabric pocket positioned on an exterior surface of the flexible ball bag; and,
FIG. 2 is a bottom, rear and right perspective view of the flexible ball bag of FIG. 1.
The broken lines showing the shape of an article is for illustrative purposes only and forms no part of the claimed designed. The characteristic feature of this design resides in the appearance of a carrying handle oriented substantially perpendicular to a zipper and the design may include at least one open mesh pocket positioned on an exterior surface of the flexible ball bag giving an appearance of fullness and bulk to the flexible ball bag.