A dual-winged slat having a rectangular body further possesses a straight fin and an angled fin on each edge of the body with the free end of the angled fin being angled toward the face of the body which is farther from the place of attachment of the angled fin to the edge. The free end of each straight fin must be closer to the face of the body nearer where the first end of the straight fin is attached to the edge than is the free end of the angled fin on the same edge. No fin may be attached to the edge of the slat more than halfway to the face toward which the angled fin is directed. The fins are preferably asymmetrically arranged. And, also preferable, the straight fin and the angled fin attached to a given edge are attached at the same place.