The ornamental design of an apparatus for supporting a stringed musical instrument, as shown and described.
FIG. 1 is a top perspective view of an apparatus for supporting a stringed musical instrument embodying my design;
FIG. 2 is a front perspective view showing the apparatus for supporting a stringed musical instrument in full lines with the broken line showing of environmental association matter being for illustrative purposes only and forming no part of the claimed design;
FIG. 3 is a side elevational view of the apparatus for supporting a stringed musical instrument in full lines, and the environmental matter shown in broken lines; and,
FIG. 4 is a back perspective view of the apparatus for supporting a stringed musical instrument shown in full lines and the environmental matter shown in broken lines.