An apparatus includes a top wall and a bottom wall. The top wall and the bottom wall define and enclose a first fin area; a second fin area; and a flex region joining the first fin area to the second fin area. The flex region is connected in fluid communication with the interior of the first fin area and the interior of the second fin area. The flex region is bent or twisted at a nonzero angle around at least one axis relative to the broadest surfaces of the first and second fin areas.