I want to make a small mold I would later use to create a fiberglass
container to hold lead weight. It would be nice if mold material can
withstand 500 degrees Fahrenheit so I can pour lead into it and then
cover it by fiberglass. Alternatively I would use the mold to create a
fiberglass container first and fill it up with lead pellets. So my
question is what do I use to make a mold, gypsum if so what kind,
anything else?

Andrzej