The ornamental design for plate for the sound hole of a guitar, as shown and described.
FIG. 1 is a top plan view of a plate for the sound hole of a guitar showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right-side view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a front view thereof, the broken lines are shown for the purpose of illustrating environmental structure and forms no part of the claimed design.