I claim the ornamental design for a seat belt cushion, as shown and described.
FIG. 1 is a perspective view of a seat belt cushion, shown in a position of use with a user and seatbelt;
FIG. 2 is a top view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a back side view thereof;
FIG. 6 is a front side view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines depicting a user and seatbelt in FIG. 1 represent environment that forms no part of the claimed design. All other broken lines depict stitching that is part of the claimed design.