FIG. 1 is a front right perspective view of a seat cushion illustrating the claimed design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side elevation thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof.

The broken lines in the drawings are for illustrative purposes only and do not form part of the claimed design.