Step 17: Bookholder: Second Strip.
Set your miter box in place. This miter box is 13.5x7". Set the next strip of plastic in front of it and push it against the miter box so that it becomes a kind of guide rail for miter box motion left and right.
Give it a dab of hot glue to hold it in place while you adjust it. While the glue is still hot, gently push the plastic against the miter box while moving the miter box back and forth. The idea is to get the miter box to move freely, but not to have much "play" if you try to rotate it. The smoothness of this mechanism can really make scanning more or less fun, so spend a little time on placing it right.