A housing having exterior lateral side walls, top and bottom side walls and a single internal wall running adjacent to a lateral side wall. The housing defines an internal rectangular compartment and a parallel side track between the adjacent internal and external walls. A collapsible bag filled with liquid and a slide member, actuable by a constant torque spiral spring are located in the compartment. The slide member has a boss fitting and cooperating with the track to allow the slide member to freely slide without a rotative component under action of the spring.