When you write on the glass, you can't see the direction of the gesture, and you can't see anything on its edge, even wool paper. If ink seeps in, it will soak the copybook. In fact, there is a transparent condom for learning calligraphy, which is specifically aimed at your situation. I wonder if you have it there?
Actually, you can make one yourself. You can refer to the briefcase and buy two or three pieces. See that transparent one? Isn't it sealed on all sides? If you cut the long side from the inside and the short side from the inside, you can put the copybook inside and cover it with tassel paper, so that you can see it clearly without damaging the copybook. Note that the briefcase must be smooth and transparent!
I hope it helps you!