This PetSafe Staywell Deluxe Tunnel Extension is a great addition if you have thicker doors or walls. It extends your cat flap by an additional 48 mm and is compatible with Staywell Cat Flaps (300, 400, & 500 Series). The PetSafe Staywell Deluxe Tunnel is easy to assemble and incredibly robust, making it a great solution for helping your cat get from outside to inside with ease.