The ornamental design for a box side panelassembly, as shown and described.
FIG. 1 is a front perspective view of a box side panel assembly showing our new design;
FIG. 2 is a side view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
It will be understood that the broken lines shown in the drawings illustrate portions of the box side panel assembly that form no part of the claimed design.