A little more information would help. What size box do you need? What size cereal box are you cutting up? Please give all three measurements - Height, Width, Length.
If you have a Michaels near you They carried a box maker from Martha Stewart I don't see why you cannot make your own It is a piece of plastic with grooves cut in it and you select the groove to determine the size of your box, place your material on the surface and run a ball point pen or stylus on your material over the groove this will put a crease where you want to fold it. I don't see why you could not take a piece of plywood cut a groove the thickness of the saw blade to your particular dimensions using the old-fashioned handsaw.
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