I often suggest that students view their subject matter through a simple viewer to isolate composition options. To make an adjustable viewer, use an exacto knife to cut out a 2″ x 2″ window in the center of a 6″ square piece of mat board. I like this size because it is large enough to hold comfortably and the window is small enough to look through at a comfortable distance from the subject.
To make the window adjustable, cut a vertical piece of mat board 6″ long by 2.5″ wide to use as a slider. Place a rubber band over the window and the slider. Then you can adjust the slider to change the proportions of the opening.