The ornamental design for a flexible ball bag, as shown and described.
FIG. 1 is a top, front and left perspective view of a flexible ball bag depicting a basketball or baseball showing a carrying handle oriented substantially perpendicular to a zipper;
FIG. 2 is a bottom, rear and right perspective view of the flexible ball bag of FIG. 1;
FIG. 3 is a top, front and left perspective view of a flexible ball bag depicting a soccer ball showing a carrying handle oriented substantially perpendicular to a zipper; and,
FIG. 4 is a bottom, rear and right perspective view of the flexible ball bag of FIG. 3.
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.