A freezer insert, a warmer insert or a freezer/warmer insert designed for use inside plastic food containers conforms to the bottom of the various sizes and shapes of the most popular plastic food containers sold under brand names such as Ziplock, Glad, Tupperware. The insert is a flat disk and contains known gels or thermal preservation fluids useful for portable cooling or warming. The insert also can rest on the walls of the container if those walls slope outward in the direction of the opening. The inserts divide the container vertically into cool, extra cool or hot compartments for different types of food. The disk can be round, square or of other shape. The inserts are stackable to allow efficient storage, for example in a freezer or kitchen cabinet.