Seated rowing with the humerus tracking in the horizontal plane. This means using a straight bar, not a triangle bar. Also try doing a pull-down with no elbow or shoulder movement, only shoulder girdle depression. It’s the best way I know to isolate the lower trap contribution to scapular depression. Also, doing horizontal reverse flyes with dumbbells while lying prone on a bench. Keep the humerus at ~45 degrees of shoulder abduction.