How do you make a dog door air tight?

Good info here:

https://grist.org/living/your-doggie-door-is-probably-wasting-energy-but-you-can-fix-that/

https://www.doityourself.com/stry/how-to-insulate-a-pet-door-flap

The best way to fight that problem is to get the dog door that is made for weather extremes to combat the air coming in. The literally have an inner and outer flap. We have one for our dogs (we have four from 15 to 100 pounds). We were worried about getting an extra large one and having wind and drafts come in our back door, so we went with the two flap weather system and have very little problem with cold air. Even the small dog can get through the flaps, although if the larger dogs know she is coming, the bigger ones hold it open for her. We found this system works much better than the single flap and for some reason the flaps don't rip as easily at the top either. We have had ours for three years know and the flaps are going great. Most of the dog doors come with a door you can put up when it isn't being used, perhaps using that would help with the air coming in. If you can find something flexible enough, you could perhaps put that around the flap to fill in the little gap caused by the size of the flap vs. the opening.

I am a pet sitter and I go to homes where there is two flaps that are extra thick. I sit at the front door left open and the dog door is at the storm door and a little draft but not much... Hope it helps??