The ornamental design for a boot lower side panel, as shown and described.
FIG. 1 is first side elevational view of a boot lower side panel embodying my new design, the opposite side elevational view being a mirror image thereof;
FIG. 2 is a perspective view thereof looking from above and the rear on the first side, the perspective view on the opposite side is a mirror image thereof; and,
FIG. 3 is a perspective view thereof looking from the front and above on the first side, the perspective view on the opposite side being a mirror image thereof.
The outline of a boot in broken lines shows the environment in which the panel of the claimed design is located and forms no part of the claimed design.