I cut off the back of a black plastic folder since it was almost the right size. Definitely matte and with a very fine texture that makes it very light absorbing. If you shine a flash light on it there is a very diffuse circle. Probably not quite as good as construction paper, but very close. I was looking for something that is thin enough to be inserted into a magazine without deforming the pages at the edge but letting no light through.
And yes, what I did in this case doesn't scale. That's why I started this thread. And to give you an idea of how bad the paper quality of this magazine is: You can see the bleed through from the next page with your naked eye from any angle. You don't need to shine a light on it. If it weren't for the subject matter, I wouldn't waste any effort on these magazines
