Our window draft stoppers are filled with a mix of perlite and sand. The perlite is commonly used in construction as an insulating material and the sand gives them just enough heft to hug your window sash and sill. They are made using an inner tubing that will not allow the sand or perlite to leak out and are covered by a white cotton cloth to blend in to the window. The tubing is not filled completely so that there is enough “play” in the draft stopper to allow for window locks or to go up the sides of the window an inch or so if your window is not exactly the same size as the draft stopper. The white cotton covers can be washed periodically if needed, although regular vacuuming would be enough to keep them clean. You could also make covers out of the same (or contrasting) fabric as your window treatments to add to the overall décor of your room(s).