https://securityholes.science/....wiki/Cat_Flap_Fitter
It is possible to install the cat flap inside the door panel made of upvc or double glazing glass sealed units, although the best option is to purchase a brand new pane of glass with an already-made hole. Read the instructions that should be provided with your cat flap kit and follow them carefully. Make sure that the drawing accurately represents the shape (circle or square). Safety If you have a dog or cat and want to allow them access to the garden without leaving the door open A microchip activated pet flap inside your door panel