The ornamental design for a back support cushion, as shown and described.
FIG. 1 is a front elevational view of a back support cushion showing my new design
FIG. 2 is a right side perspective view thereof, the left side perspective view being a mirror image; and,
FIG. 3 is a right side view thereof, the rear being a flat surface.
My back support cushion is characterized by an elevated elongated convex shaped middle section with two adjacent elongated flat surfaced structures, and one convoluted section centrally positioned at the top end.