The ornamental design for a portion of storage container, as shown and described.
FIG. 1 is a perspective view of a portion of storage container showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is an exploded rear view thereof, the left and right side views being identical thereto; and,
FIG. 4 is a top view thereof.
The broken lines in the drawing are for illustrative purposes only and form no part of the claimed design. The shading on the portion of the lid indicates that it is opaque and the shading on the bottom portion of the container indicates that it is translucent.