The ornamental design for a cover for a lock box, as shown and described.
FIG. 1 is a perspective view of a cover for a door lock showing my new design; the broken lines showing of a padlock, doorknob and door are for illustrative purpose only and form no part of the claimed design;
FIG. 2 is a top plan view of a cover for a door lock; shown in an expanded position;
FIG. 3 is a front elevational view thereof; the rear elevational view being a mirror image thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a right side elevational view thereof; the left side elevational view being a mirror image thereof.