I tried to find something like this in the pet
department of the big store I was in last week and struck out.
It's just a compilation of 5 scrap pieces of wood and a folded over
flap of screen material. A notch on the right side with a dab of glue
seems to be enough to anchor it to the screen frame. I hope our cats
are smart enough to adapt to a proper pet entrance which can be easily
closed down at night by shutting the window.