The ornamental design for an insert for a container, as shown and described.
FIG. 1 is a top, front and side perspective view of a first embodiment of an insert for a container embodying my new design.
FIG. 2 is a bottom view thereof.
FIG. 3 is a top view thereof.
FIG. 4 is a side view thereof
FIG. 5 is a side view thereof opposite the side view of FIG. 4.
FIG. 6 is a front view thereof.
FIG. 7 is a back view thereof; and,
FIG. 8 is a reduced scale, environmental view thereof.
The broken lines showing portions of the insert are for the purpose of illustrating environment and form no part of the claimed design. The broken lines showing the container are also for the purpose of illustrating environment and form no part of the claimed design.