Abstract: A reptile cage liner is formed as a blank having selective cuts and creases to reduce section modulus to provide a stiff but foldable wall (flap) on each side of a rectangular floor. Flaps fold or bend preferentially along creases to form the liner. Residual stiffness of creases urges edges of the flaps to seal by contacting walls of a cage tray. Corner, funnel tabs tilt downward and extend laterally from each side flap across a corner gap to prevent escape of bedding, food, or other debris into corners or behind the liner. Liners may be easily replaceable and disposable.