I went to work and made my covers with some "experiments" in my box. They turned out fine. Got the paper signatures made and put everything together to see the final appearance and discovered that the covers were not the same thickness. I wanted to make sure the front cover which was made out of cut up clay sheets would be solid enough:in other words, it would not be flexible. That cover is thick. Now I have two books to make, one with thicker covers and the other with thinner ones. If you look at the shadows cast by the covers you will see this.
Not to worry, I have a good idea for the second book. More play for moi!
TIP: it is not a good idea to drink tea while painting with watercolours!