A Plexiglas tube equipped with weights that can be located anywhere between the center of the tube and the two ends.
Thumbscrews hold the weights in place.
With the weights at either the center or the ends, rotate the tube back and forth about an axis perpendicular to the tube. Move the weights to the other extreme position and compare the torque required. Even though the difference probably will be obvious to the class as the instructor rotates the tube, it perhaps will be more convincing to let a student do the rotating, or maybe it could be handed around for several students to try. Those using Halliday, Resnick, and Walker will perhaps recognize this demonstration as a variation of Figure 11-11 of the fifth edition of that textbook.