The bubble for the Ultra is as simple as it gets, it's just a true bubble that gets cut down to size. For more complex bubbles, like the bubble on Starbird's "Forcasta", a little more engineering goes into the tool. Here is a tool for a shaped bubble. Notice how the opening at the rear of the tool is cut to form a double-bubble effect. To set height and add contour, a restrictor is placed down the middle (note arrow). When the bubble is blown it will rise up and hit the restrictor. As the bubble rises it will blow up and around the restrictor giving the bubble its shape. A tool like this will give the double effect like the Forcasta's.