We had a project come in last week that required building a new home for the overstuffed documents of one store-bought 3-ring binder. The ring mechanism itself was up to the task, but the boards were too short and the spine was too narrow; the whole book had taken on the shape a flying-wedge.
The hardware was removed and prepared thusly.
Rich had the brilliant idea to use a section of a giant tube mailer to create the new spine piece. He also had the table saw and steady hands to cut it. The piece was cut to accommodate the thickness of the book's innards, and provide enough room at head and tail for the page dimensions.
The thing weighs a ton. But now it lies flat and I think it's bullet-proof.