My DIY, homemade pocket sketch book fits into my shirt, pants or jacket pockets. (outside dimensions are 4½ by a little over 3 inches or 115 by 80 mm). This neatly fits over and protects the paper which is A6 size. I use an A4 sheet of standard copy paper folded along the long side and torn (too lazy to cut), then folded long side again and torn, then folded in half to make the leaves.
The sketchbook has a watercolour pencil (sepia) attached, so that I can always sketch using line and tone. because it is a watercolour pencil, I can wet the shading to create a watercolour effect.
The pages are removable, so I can always have clean paper to work on.
The front and back covers are separate and hinged to the bamboo stick using filament tape, which is also what I used to construct the pencil holder.
I also put a small pocket on the inside cover for my art shop discount card.
Given my hamfisted approach, the sketch book has proved remarkably robust. I've been carrying it daily for about a year now, with no signs of serious deterioration.
I hope that this inspires you to design and build your own. I'm sure that you can do a more elegant job.
Feel free to comment and share your own solutions.