I bought the Petsafe Staywell 300 4 Way Locking Cat Flap and 7 of the 310 White Staywell Tunnel Extension For Walls to get through a 37cm wall. I emailed CatFlaps.co.uk first for info and they assured me this was exactly what I needed. My package came in 48hrs and worked a treat. Once the initial hole had been knocked through (I looked on YouTube for some guidance) the extensions were glued and duct taped and it all just slotted together. Very pleased with the service, price and product (best price I could find online!). Just gotta persuade the cat to use it now. Thanks guys!