The ornamental design for a mouthplug for container, as shown and described.
FIG. 1 is a front perspective view of a mouth plug for container according to our design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dashed lines in the drawings depict environmental subject matter only and form no part of the claimed design. The long dash-short dash-long dash lines form boundary portions and form no part of the claimed design.