I use a green/brown cloth drawstring bag that used to hold balls of yarn and similar stuff. I found it at a second-hand store, bought it, gave back the yarn, and kept the bag. It has a circular bottom with cardboard or something to stiffen it, and a cylinder attached to that with drawstrings on top. The bottom diameter is about 17 cm (7"), and when open it's about 19 cm tall (7½").