Best one I have ever seen is this one https://www.thepetflap.com/, it gets a mention in my book, it can’t blow open in the wind!

Yes, they're terrible. Neighbours have a Petflap and it's definitely airtight. However, the big potential issue is that it has no lock so theres nothing to stop other animals coming in through it as well as your cats.

We ended up just fitting a Catmate but, because triple glazing/large sliding door ended up putting it through a wall section rather than a window so ended up with a very long tunnel. It's definitely not airtight - the lock means it's not going to blow open but you are relying just on a brush seal and it's a single piece of plastic. I reckon a two flap airlock is probably the best solution if you need one that locks - or into some intermediate space like a garage before going outside.

We have two cat flaps. The inner one is mounted in our house wall and is an airtight one from Freedom Pet Pass in Canada. It works well but is a little bit stiff for cats to use, so they can be reluctant sometimes. Outside that cat flap is a space we call the 'bin store' which is just an enclosed area built without insulation and used for mostly food storage. In its outer wall there is a second cat flap, which is a conventional transparent cat flap with microchip sensor. It prevents neighbourhood cats getting in and our cats like to sit just inside it looking out.

Yes, we chose it for that reason, and the price was a lot more sensible than the PH-certified Austrian jobbie.