Measure the circumference and height (minus the leaf tops) of the largest stalk of celery in the garden.
Cut rectangles out of your cardboard box. Make the length equal to the celery stalks' measured circumference plus 4 inches. Make the height equal to the height of the measured celery stalks.
Roll the cardboard rectangles into tight tubes to make them more pliable and encourage them into a cylindrical shape.
Place a cardboard cylinder loosely around the stalks -- with an inch or so of space around the stalks -- of each celery plant. Tape the cylinder closed with a length of duct tape.
Remove the cylinders at harvest time.