This paintable tunnel extender from Closer Pets can be added to compatible pet flaps to achieve the desired wall thickness. Each liner section measures 50mm (2 inches) in length. You can combine multiple sections to create longer tunnels, for instance, four sections would create a 20cm tunnel. Sections can also be cut to size if a specific length is required, such as a 22cm tunnel using five sections cut down. For moisture resistance, a thin bead of sealant can be applied between sections, and wrapping the outer joint with duct tape provides extra stability and easier handling. This extender is compatible with Closer Pets Large Cat Flap (Ref. CP221W), Cat Mate Large Cat Flap (Ref. 221W and 221B), and Cat Mate Large Glass Fitting Cat Flap (Ref. 357W).