An elongate structure that includes a hollow outer portion and a hollow inner portion disposed within the hollow outer portion. The hollow outer portion includes a first outer side surface, a second outer side surface, and a third outer side surface that define first, second and third intersecting planes respectively. The three intersecting planes form three dihedral angles whose sum is about 180°. The hollow inner portion is in contact with each inner side of the hollow outer portion. Further, the hollow inner portion includes at least one panel that extends between each inner side of the hollow outer portion to thereby form hollow inner triangular channels.