A shell assembly configured for holding one or more weighted articles includes an outer shell having an outer surface and an inner surface defining a cavity configured for receiving the article(s) therein, with an opening providing access to the cavity, a closure configured for selectively opening and closing the opening, and a securing structure configured for securing the article(s) within the cavity to limit movement of the article(s). The securing structure includes a plurality of adjustable straps each connected to the inner surface of the outer shell at a first securing point and a second securing point on opposite sides of the opening. Each of the straps is adjustable with respect to the outer shell to increase or decrease an effective length of the respective strap between the respective securing points, to increase or decrease a distance between the respective securing points and thereby constrict or expand the outer shell.