A polymer currency cassette for an ATM. The cassette includes a one-piece polymer body attached to a one-piece polymer lid. The lid is arranged with the body to resist entry into the cassette interior. Integral stiffening ribs are provided. The body side walls include integrally formed guide tracks. A one-piece tambour door is securely movable along the guide tracks. The upper edge of each body side wall extends into a gap created between double-walled side walls of the lid when the lid is closed. A tab, which projects outward from an upper edge of a body side wall, extends even further into the closed lid. The tab becomes nested in an aligned slot in the lid when the lid is closed. A nested tab affords further cassette security by providing resistance to lid and body separation.