A fluid warming cassette with a stiffening frame structure and a key mechanism is provided to support a parenteral fluid container. The fluid container is desirably thin to minimize heat exchange inefficiencies. The frame structure permits the thin fluid container to be inserted into the narrow space between fixed position warming plates of a warming unit. The frame structure has a quadrilateral shape with sides and ends. The fluid container is attached, at its periphery to the sides and ends of the frame structure, within the quadrilateral shape. The key mechanism prevents the cassette from being inserted upside down or backwards in a warming unit. Part of the frame structure may be formed into a handle to assist in both the insertion and removal of the cassette from a warming unit.