FIG. 1 is a perspective view of a rectangular bell structure of the claimed design taken from the front, top, and right side;

FIG. 2 is a front elevational view of the rectangular bell structure thereof;

FIG. 3 is a rear elevational view of the rectangular bell structure thereof;

FIG. 4 is a left side elevational view of the rectangular bell structure thereof;

FIG. 5 is a right side elevational view of the rectangular bell structure thereof; and,

FIG. 6 is a top plan view of the rectangular bell structure thereof.

The broken lines shown in the drawings are for illustrative purposes only and form no part of the claimed design.