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 Brandon Hilton

First off, this well made cat flap can't be beat for the price. Alone, this works best on a thinner solid wood door. If you want to use it to install hollow core door or wall, you can buy an extra piece to fill the gap between front and back. My door panel was about 0.5" thick MDF and the front and back matched perfectly. I decided to hide the litter box under the second sink in my bathroom. I've never used a jigsaw so this was the hardest part, especially as I had to work around the door skin but once the hole was cut to the template provided it was easy to line up the two sides.The back is actually larger and fits snugly in the hole.You'll want to put it in place and slide the front in before you mark the screw holes on the front. The back uses only double sided foam tape. We'll see how well this holds over time, but I can always reinforce the door with silicone. I taped the door to get the cats used to it before I started getting them in and out and added a motion sensor LED light to let them se know what they are doing. Deals with stinks/mess really well and keeps my 3 dogs out of the trash!