Spray-paint the foam ball; let dry. Insert the dowel into the foam ball. (If desired, cut one end of the dowel to a point to make it easier to insert.) Remove caps from ornaments.
Cover the neck of an ornament with hot glue, then carefully press the ornament into the foam ball. Repeat with the rest of the ornaments, turning the foam ball as you work to determine the best placement of various colors and sizes. (Work in a random pattern rather than trying to work in rows, or cover one side of the ball first.)
When the foam ball is covered, fill in tiny gaps with rhinestone picks or other prewired filler. Tape ribbon to the top of the dowel. Tightly wrap ribbon around the dowel, stopping about 3 inches from the bottom and taping the end to secure.
Tie a separate piece of ribbon around the top of the dowel, leaving two tails, each about 7 inches long. Notch the ends of the ribbon tails.
Fill a vase or container halfway with modeling clay. (If you plan to reuse the clay, cover the top with plastic wrap so it doesn't dry out.) Insert the dowel into the clay. Cover the clay with marbles.