Foil tape is exactly that, metal foil with an adhesive backing. A very thin metal foil, backed by paper containing fibers or string for strength, which is in turn backed with an adhesive. The aluminum soda cans are made from is thin enough to be easily worked with. Its edges are sharp, however, and can cut skin deeply, so be careful when using this material. Its thickness is good for projects where structural integrity is required.
Spherical shapes are common to electrostatic devices for a couple of reasons.
Another is that the larger the radius of a charged sphere, the longer the spark is that can be drawn from it. There are many varieties, the rounder and smoother, the better. Be aware that some knobs may be round but have sharp edges near their base. This may not disqualify them for use, but could be a deciding factor if you have several to choose from. Also be aware that some metal knobs have a clear plastic coating, avoid those.