Weave around a Styrofoam cup.
Round reed can be inserted across the cup near the bottom. A base can be twined for a couple inches or start weaving up the sides right away, using the shape of the cup as a mold. After the weaving is finished and secured, break away the Styrofoam. Voila', a finished basket.
You can rewet the basket and push the bottom up so it sits flat.