Wax-based compositions for making barrier layers used in oral treatment devices are thermally stable when formed into a flat sheet or three-dimensional article to a temperature of at least 45° C. and are plastically deformable at room temperature (25° C.). The wax-based compositions include a wax fraction homogeneously blended with a polymer fraction. The wax fraction includes at least one wax and the polymer fraction includes at least one polymer selected such that, when the at least one wax and at least one polymer are homogeneously blended together, they yield a wax-based composition having the desired properties of thermal stability and plastic deformability. Barrier layers in oral treatment devices made from such wax-based compositions are dimensionally stable to a temperature of at least 40° C. without external support and can be plastically deformed in a user's mouth to become at least partially customized to the size and shape of user's unique dentition.