You guys have no imagination.
You can use:
Tinfoil, Folded and pressed till its the right thickness, and then pressed to remove creases.
Lead foil, avliable from some Hobby shops, or a plumber, Or even better the top of a champain bottle.
Very thin strips of card, softened in water, shaped then let dry.
Insulation from around 240V house wiring. (the flat retangular stuff)