The ornamental design for a musical instrumentcushion, as shown and described.
FIG. 1 is a front elevation view of a musical instrument cushion, showing my new design;
FIG. 2 is a back elevation view of the musical instrument cushion of FIG. 1;
FIG. 3 is a left side elevation view of the musical instrument cushion of FIG. 1; the undisclosed right side elevation view being a mirror image thereof;
FIG. 4 is a top view of the musical instrument cushion of FIG. 1; the undisclosed bottom view being a mirror image thereof;
FIG. 5 is an isometric perspective view showing the musical instrument cushion of FIG. 1 in use;
FIG. 6 is a front perspective view of the musical instrument cushion of FIG. 1; and,
FIG. 7 is a back perspective view of the musical instrument cushion of FIG. 1.
The broken lines in FIG. 5 illustrate the musical instrument cushion in use and form no part of the claimed design.