Hmmm… I have had this problem in the past so I usually use computer generated slant lines and a light table. This also happens to me if I try to make my own guidelines in Photoshop. Sometimes about how the line shifts as it goes across the page.
Do you have access to one of the templates available through Paper & Ink Arts? It’s a template made from acrylic that you place over the page. I think it has slant lines as well.
Hoping someone else pipes in with some ideas.
Regarding copyright. As soon as you create something, it is automatically “copyrighted” as it is your work. However, in various countries, you can register the copyright of your work. This is especially important if you were to ever have to defend your copyright later down the line. I haven’t searched the US process in some time. But I suggest searching your country (if not in the US) and copyright process.