This insert is made to sit in a window. The flap allows a cat access in and out of the window without letting cold/hot air in. To complete the assembly as shown in the picture requires a couple of additional items.

Transparent plastic was glued to the back of the Flap and Sides with openings. This is what I used but any plastic or vinyl sheet would do.
[https://www.amazon.com/dp/B09FY67B97](https://www.amazon.com/dp/B09FY67B97)

And lastly a board was used to fill the space next to the pet door.