Some of you may have seen this but for those who haven't these are videos on making an Art Journal out of a composition book and manilla folders. Its done by Samantha Kira. You could easily substitute watercolor paper if you like that better.
I haven't made one this way but it looks pretty simple. The ones I have done are using canvas boards as covers but I put it together the same way. I think all Art Journalers at some point start making their own books. They are relatively easy to do and then you can make it any size and also add the paper that you like.