Seems you need a catflap installer! The one to use is :
Four Paws Doors
who's an accredited installer named on the Sureflap (SurePetCare) website (Sureflap make high end cat & dog flaps), although he'll install all brands (as far as I know). He fits them thru wood, glass, brick, you name it, & did an excellent job for me a couple of weeks ago with a new Sureflap Microchip catflap. In my experience, it's best to buy a top quality brand of catflap (altho not necessarily cheap) & use a reputable fitter. You don't want other cats breaking into your house, nor do you want your cat locked out on these cold nights! Good luck.