An arch-shaped mouthpiece for forming custom dental trays intraorally, which is typically comprised of a sheet of low-melting custom tray material containing a filler, preferably air. The mouthpiece may also have a supporting frame comprised of a facial wall connected to a lingual wall, and a handle. The supporting frame may support the tray material and filler for a single arch, or for both arches. The tray material may contain occlusal pressure indicating means, such as small, rupturable capsules. In typical use, the mouthpiece is heated, the teeth forcefully bite into the tray material and contained filler, thereby molding the tray material to the teeth, the tray material is cooled, the mouthpiece is removed from the teeth, and excess tray material is trimmed away to form a tray.