So far,so good.
The box seems to be holding heat well enough to keep fragile seedlings from freezing at night. I don't water it on the cold days ( under 50 with no sun) because we get freezing temps at night.
Its holding moisture well too. I thought the cardboard might suck out too much moisture but it isn't.
The second box isn't doing as well. Nothing is growing and its right next to the other one. That box has a small hole in the top about the size of a fig. I think too much heat escapes. I'm going to add a second layer of plastic and see how things go.