The ornamental design for an insert for a container for cupcakes, as shown and described.
FIG. 1 is a perspective view of the insert for a container for cupcakes;
FIG. 2 is a top view thereof, shown with the flaps of the cupcake receiving aperture in an unfolded configuration;
FIG. 3 is another top view of the insert shown in FIG. 1;
FIG. 4 is a bottom view of the insert shown in FIG. 2; and,
FIG. 5 is a right side view thereof, the left side, front side, and rear side being a mirror image thereof.
The broken lines showing a cupcake illustrate environmental structure. The broken lines immediately adjacent to the aperture flap illustrate fold lines. All other broken lines illustrate portions of the insert for a container for cupcakes. The broken lines form no part of the claimed design.