Cathole ECO Cat Door, White
I've been buying these Cat-Holes for 10+ years now, to put on doors in various houses I've lived in. And they never fail to deliver! You do have to have some mechanical know-how to install it, obviously, and a jigsaw (or even a handsaw, or something that will cut in a circle pattern). If you're looking for the white plastic model, as shown here, do not order it from Shamazon, they'll send you the wrong model (the wooden/brown colored one!) Chewy did a great job and shipped this to me lightning-fast! Highly recommended.