a body having an upper end and a lower end and a plane cross-sectional shape of a circle, the circle being out-of-round over substantially an entire height of the body, the body including:

crests disposed at three or more points on a circumference of the circle at substantially a same interval, wherein a central angle (α) position of each crest fluctuates similarly vertically along the height of the body;

support ridges formed by the crests in sigmoid curves and disposed at least in three substantially parallel rows at a same interval; and

panels disposed between adjacent support ridges and provided with slightly swelled panel walls that are reversibly deformable into a dented shape, as seen in cross-sectional plan views.