I also needed to make some drawers to hold stamps which will be accessed from the side of the book. I usually would use Altoid tins for this, but they are too deep and were taking up 1/3 of the thickness of the book. Cereal boxes folded into flat boxes and covered with paper work fine.

They go in the side of the book. See the gap where one will slide in on the book at the bottom of the stack?