For wall installations, tunnel extenders can be stacked together to create a longer approach tunnel to the SureFlap Microchip Cat Door, the DualScan Microchip Cat Door or the Microchip Cat Door Connect when it is installed in a wall. The Cat Door Tunnel Extender fits snugly onto the 70mm long tunnel of our cat doors. Each tunnel extender adds 50mm to the overall length of the tunnel.