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My Mom did something like this years ago, but she used a single piece of cardboard cut long enough to be back, bind and front. However she always said this could only be used on magazines that were stapled. If the magazine was glued the string would create a pressure point that would break the magazine's binding over time and i noticed the magazines you show are the stapled versions.
Link to where to purchase kit including a PCBhttp://jasonkits.bigcartel.com/
XYZ Davinci sold thru Amazon and other places.