https://telegra.ph/How-to-Inst....all-a-Cat-Flap-11-08
It is possible to install an apron flap inside an upvc panel or double glazing sealed unit. However, the most effective option would be to install a new glass pane with a hole that is already made. Follow the instructions provided with your kit. Ensure that the shape drawn (be it circle or square) is accurately drawn on your template drawing. Safety A microchip-activated pet panel inside your door made of upvc is a great option if you have a dog or cat and wish to let them out into the garden, but do not want to leave the door open.