The ornamental design for a lighted letter panel, as shown and described.
FIG. 1 is a front view of the panel, according to the present invention;
FIG. 2 is a left side view of the panel, according to the present invention, the inset is an illustration of the tabs according to the present invention;
FIG. 3 is a right side view, according to the present invention;
FIG. 4 is an isometric view of a corner section according to the present invention; and,
FIG. 5 is a rear view of voids to accommodate a male snap, according to the present invention.