Clear latex balloons, condoms and gloves are opaque until they stretch. The focal length on these spheres is short, about one to two inches, entirely depending on the size of the balloon.
The more sphere shaped it is, the better it will work. If the balloon is too big, it becomes a squashed sphere and it’s focal point becomes distorted, like a line instead of a tight circle. Sometimes just giving the balloon a squeeze in the right axis will bring the light back to a nice tight circle.