It is claimed the ornamental design for the sign stand, as shown and described in the accompanying figures.
FIG. 1 is a perspective view of the sign stand according to the present invention;
FIG. 2 is a front side elevation view of a sign stand of FIG. 1;
FIG. 3 is a back side elevation view of the sign stand of FIG. 1;
FIG. 4 is a right side elevation view, of the sign stand of FIG. 1; the left side elevation of the sign stand being a mirror image of the right side elevation view;
FIG. 5 is a top plan view of the sign stand of FIG. 1; and,
FIG. 6 is a bottom plan view of the sign stand of FIG. 1.