I claim the ornamental design for an underside of a cargo box lid, as shown and described.
FIG. 1 is a perspective view of an underside of a cargo box lid;
FIG. 2 is an alternate view thereof;
FIG. 3 is a front elevational view thereof; and,
FIG. 4 is a cross-sectional view thereof.
The sectional view FIG. 4 is included to show the contours of the design. No claim is made as to the internal construction of the design. In the drawings, the broken lines depict portions of a cargo box and form no part of the claim.