My latest project is to fit a microchip pet flap to this entrance door at my daughter's house. It is the only suitable door.
Black - cutout hole. Red - plastic surround. Yellow - wooden fill-in blocks.
Wondering if cutting away part of the lower mullion and a bit of the bottom rail would seriously weaken the door. Pretty sure a decent kick would break the frame by the windows anyway.
I have fitted the flap to a piece of ply and wedged it in a sliding door. The door is taped open to get Tom used to going through it. Luckily, he went straight through without batting an eyelid.
Will see how it goes when I put the batteries in the flap is operating.