I have just measured the tightly assembled front and rear: approx. 2.4 cm deep; i.e. the door should be at least this thick, otherwise the whole thing will protrude. However, the assembly does require a bit of "tinkering" because you have to saw the cut-out in the door and the whole thing also has to be attached to the corners of the cut-out. So you have to mark the size on the door, define the drill holes and work quite cleanly so that it looks nice afterwards.
The packaging states that the "maximum installation depth is 13 mm". This is not correct, because you can use the
You can simply screw the front of the flap (with the flap and the closing mechanism) to one side of the door and the back to the other side, with air in between.
We have provided a wall opening (approx. 20cm deep) with this flap on the outside and it works perfectly.

As the flap had to be fitted into an existing door panel ("balcony door" in PVC according to Bauhaus), I cut a board (15 mm) to size for the panel and then made the cut-out for the flap in this board. It works well in this thickness.
My comment is that the whole thing works somewhat awkwardly if you want to close the flap outwards and keep it open inwards. Press the flap faster so that you can catch it, when it swings back, press the red slider (right) against the centre, then it is closed from the inside to the outside. At first I had the flap the other way round, but then you have to go outside in the rain and move the slider button. We also have two cats who can easily pull on the side with the middle*finger* of their paw and get out that way. Mounted the other way round, they can no longer reach the edge.

The magnets are too weak in strong winds (not storms), so the flap hangs slightly open inwards or outwards. Then I had a look and the small magnet on the frame was broken out; of course; but we don't have fighting cats, just normal animals, and the in/out movements are also normal. So I stuck it back on. After 3 days.

The colleague below describes it very well. It involves employees.