A closure includes a plastic shell having a generally flat base wall, a peripheral skirt for mounting the closure on a container neck finish and an annular portion integrally connecting the skirt to the periphery of the base wall. A flexible resilient sealing liner is disposed on at least an inner surface of the annular portion. The annular portion has an inverted V-shape with an outer leg integrally connected to the skirt and an inner leg integrally connected to the periphery of the base wall such that the base wall is disposed axially beneath the annular connection between the inner and outer legs. The inner leg is resiliently flexible with respect to the outer leg upon engagement with an end of a container neck finish so as to flex the inner leg and the base wall outwardly with respect to the skirt and thereby draw the outer leg into engagement with an outer surface of the container neck finish. The inner leg of the annular V-shaped portion preferably is conical, and in exemplary embodiments of the disclosure the outer leg preferably either is cylindrical or is conical preferably at an angle equal and opposite to the conical inner leg.