PetSafe Petporte Tunnel Extension - tunnel, extension for PetSafe Petporte Cat Smart FlapPetSafe Petporte Tunnel Extension is helpful for installing the PetSafe Petporte Cat Smart Flap in very thick walls or doors. The tunnel should be placed in the opening between the inside and outside of the flap to ensure a safe, smooth finish for the area the pet moves through. It extends the tunnel built into the flap by an additional 4cm. You can add single or multiple extensions.