Dog house flap ideas?

My dog has a house, but he's not really using it, even though it's probably a nicer than average house, the reason why...is I guess that he likes to be in a closed space. He spends most of his time in a small space between a interior and exterior door of our house. Everything was nice until he learned how to scratch the exterior door, so that it opens. Now he doesn't even call us to open the door, he just goes out, every half an hour and sometimes he drags his blanket with him, all the way to the other part of the yard and he doesn't know how to return it and then cries for us to put it back in its place and of course to close the door that he opened. It can be a hot sunny day, but even then he wants the door closed.

So, since he has a dog house for years, but doesn't use it, I decided to fix it up for him. I cleaned all the dirt, put blankets in it, covered some holes in the wood that shouldn't be there, I put his food there and as a ,,temporary door" I nailed two parts of clothes on the entrance of the dog house, one on the outside and one on the inside, they mostly act as small curtains and the dog doesn't have a problem with them.

They are nice and elastic, but not heavy enough and the wind moves them around, they aren't flat as they should be, so there's some open spaces and the cold air gets inside, despite the fact that there's 2 of them, so...I don't know, instead I'd like to put something heavier, so does anyone have any ideas?

There's loads of dog flap ideas on the internet, but the problem is that 99% of them are for real house walls or doors, not for dog houses, so I don't have a lot of space to work with. just around 20x20 centimeters, I was thinking of maybe a door made of 3 parts of wood with maybe even some sort of a transparent material in the middle, so that he gets some light in the house. I guess that it has to be either elastic or made of multiple hard parts, because otherwise he could get stuck or just break the door.

yeah a wooden frame with a perspex middle should work well. You can usually buy small pieces of perspex on Ebay.