The ornamental design for a lunchbox, as shown and described.
FIG. 1 is a front perspective view of a lunch box showing my new design;
FIG. 2 is a bottom perspective view thereof
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is another front perspective view thereof, in a condition of use.
The broken line portions of the lunch box in FIGS. 1-9 represent unclaimed portions of the claimed design and form no part thereof. The broken lines of the additional lunch box in FIG. 9 illustrate unclaimed environment and forms no part of the claimed design. The dot-dash broken lines in FIGS. 1-9 represent unclaimed boundary lines and form no part of the claimed design.