The ornamental design for an under-seat container, as shown and described.
FIG. 1 is a front and left side perspective view of an under-seat container, embodying our new design, the front and right side perspective view being a mirror image thereof;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side elevation view thereof;
FIG. 7 is a right side elevation view thereof; and,
FIG. 8 is a right side perspective view thereof.
The broken lines are for environmental purposes only and form no part of the claimed design.