The ornamental design for a device for filling a bag with packing materials, as shown and described.
FIG. 1 is a front, perspective view of the device of the invention;
FIG. 2 is a side view of the device of FIG. 1, all other sides being symmetrical;
FIG. 3 is a top view of the device of FIG. 1;
FIG. 4 is a bottom view of the device of FIG. 1; and,
FIG. 5 is a perspective view of the device of the invention in place on a garbage bag showing its intended use, the garbage bag being shown in phantom lines and not forming a part of the invention.