Testing a Handmade Sketchbook Binding That Opens Flat

I am making a small sketchbook for daily drawing and want the binding to open flat without forcing the spine. The current version uses three pamphlet sections with a simple thread, but the middle pages rise near the center and make it awkward to draw across the gutter. I have tried wider hinges and thinner paper, which helped slightly, but the cover still feels stiff. I am considering a kettle-stitch binding or a flexible spine with cloth reinforcement, though I want to keep the object light and easy to carry. Please share advice on paper grain, thread tension, glue placement, and cover construction. Photos of a binding that lies flat would be especially useful, along with notes on how it behaves after repeated use. I would also like to know which choices matter most when the sketchbook must survive a bag, daily handling, and quick outdoor sessions.