The ornamental design for a stadium/arena seatcover, as shown and described.
FIG. 1 is a perspective view of the stadium/arena seat cover showing my new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a perspective view showing a gap between the cover and pocket disposed there below.
The broken lines showing an inner pocket disposed between the bottom surface of the seat cover and the webbed storage compartment are for illustrative purposes only and form no part of the claimed design.