Review on PetSafe Cat Door for Interior Doors - 2-Way or 4-Way Locking Flap - Enhancing Privacy for Cat Litter Box or Automatic Feeder - Sturdy Frame, Easy DIY Installation, Including Hardware Kit - Ideal for Pets Up to 15 lb (240 Oz) by Justin Early

I had to let my outdoor cats out of the shed and into the laundry room to escape the terrible cold. They ignore the cat house I built for them on the shed porch and I spent $40 on building materials so keeping street cats warm budget was pretty low. I bought it as an optional item for $6.06, which seems like a very fair price (and it was the cheapest cat door I could find on Amazon). She did exactly what I needed. There have been many complaints about this cat door leaving the inside of your hollow door open, but that's to be expected - this cat door makes no claims in that regard. There are more expensive cat doors that do the same thing, but in my opinion it's not worth the money they want. If I were really concerned I would take some thin flexible wood or plastic and cut the open inner core of the door or fill it with foam and cut clean. I don't care so I didn't do it. The only negative I can say about this cat door is that the "inner" bezel is held to the door with double sided tape, not screws. But if you clean the door before you apply the tape you shouldn't have any problems, or you can always drive a few screws through the panel and into the door.