The ornamental design of a panel for a suitcase, as shown and described.
FIG. 1 is a perspective view of a panel for a suitcase showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 shows a perspective view of a panel for a suitcase, in use mode.
The broken lines showing portions of the luggage and luggage handle are for the purposes of illustrating environment and form no part of the claimed design.