An exemplary shackle is configured for use with a crossbar having a pair of openings and a locking mechanism including a pair of deadbolts. The shackle includes a substantially flat plate portion having a length in a longitudinal direction, a width in a transverse direction, and a thickness along a lateral axis defining a proximal direction and a distal direction. The length is greater than the width, which is greater than the thickness. The shackle further includes a pair of longitudinally-offset legs extending distally from the plate portion. Each leg has a diameter, and includes a bumper and a notch positioned distally of the bumper. An offset distance is defined between the distal surface of the plate portion and the distal faces of the bumpers. The width of the plate portion is greater than each of the diameter and the offset distance.