The ornamental design for a lunchbox, as shown and described.
FIG. 1 is a top, front, left side perspective view of a lunch box showing
my new design; the bottle shown in broken lines for illustrative purposes
only;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a top, front, left side perspective view thereof; and
FIG. 4 is a bottom plan view thereof;
The undisclosed right side view is a mirror image of the left side.