The ornamental design of a flat topseatbackcover, as shown and described.
FIG. 1 is a perspective view showing my new flat top seatback cover;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and
FIG. 7 is a bottom view thereof; and,
FIGS. 8 and 9 are perspective views illustrating an example of the flat top seatback cover when attached to a rocking chair.
The broken line showing portions of the flat top seatback cover in the figures is included for the purpose of illustrating environmental structure and forms no part of the claimed design.