I manage a drapery and upholstery shop. Don't waste your money on "sub box polyfill." Follow crutchfields recommendations, but until they stop shafting us buy it from fabric stores, craft stores are ALWAYS more expensive, there is a difference. You can use "polyfill" if you want to stuff your box with fluff or "batting" comes in sheets of different weights (thicknesses and densities) to line your box in. Just march in to that flowered, potpourri, no mans land and ask for it.