You can use those acid free boxes, but understand acid free paper and cardboard isn't "acid free" forever. The most important thing is to keep them away from light and extremes of hot and cold. Cotton likes the same living temperatures as we do. Attic storage is the worst! Also, do not store them wrapped in plastic or touching the sides of a plastic bin, etc. because if the plastic gets damp from condensation, you'll end up with mildew on the quilts in that spot. If they are folded, unfold and refold a different way at least once a year so they don't get permanent fold lines. Don't let them come in contact with unvarnished wood because they will get brown stains from the wood over time.