I don't know about the legality, although I agree with leafylady that somewhere journals for this purpose probably already exist.
I have recently started bookmaking myself and while it is taking me some time to do each project, it doesn't take so long as to make a similar project that I might see in a store too expensive. And, it will get easier as I get more skilled. The part that takes the longest for me is choosing the materials (and finding time in my overextended days to work on the projects). Since you make your own papers, the selection stage must be easier for you. I guess I'm saying that the handmade book/journal seems to be a unique category. I'd be willing to pay good money for a beautiful and well-made one.
I've seen various sorts of kits out there for bookbinding, which include pre-cut boards and even holes for screw and post binding. Perhaps online you could find even more varieties of pre-cut boards in different sizes and for different types of binding??