A mouthpiece for forming custom dental trays intraorally having a sheet of low-melting custom tray material sealingly containing a moldable filler. The mouthpiece may also have flexible walls along the facial-buccal or lingual sides for additional support. The mouthpiece can form custom trays for both arches simultaneously, or for a single arch. In typical use, the mouthpiece is heated, opposing arches of teeth forcefully bite into the tray material and contained filler, thereby molding the tray material to fit the teeth, the tray material is cooled, the mouthpiece is removed from the teeth, and excess tray material is trimmed away to form trays for both arches.
