My current GTD lofi notebook is very simple and streamlined, and so far it is working well. I revamped it for Jan. 1, so we're still in the early infatuation phase.
I have the front 15 or so pages marked for Waiting, the back 25 or so pages marked for Projects, and the area in between is for NAs. By my eyeball, the NA section is about the same size as the Project section. The very back page is for Someday/Maybes. The sections are marked with post-it flags. If I run out of NA room because I bump into Projects, I can work backwards from the first NA page until I bump into Waiting. Similarly, if I go over a page of Someday/Maybes, I work back until I bump into Projects. Once two sections connect, time to buy new notebook. Obviously I can tweak the allocations next time based on what happens this time.
Simple, neat, self contained and easy to cart around.