I claim the ornamental design for a bag insert, as shown and described.
FIG. 1 is a perspective view of a bag insert showing my new design, the bag insert having claimed curved lines of weakness about drink receiving openings as shown in dashed lines;
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 showing the claimed curved lines of weakness in dashed lines; and,