I've seen the pro's hot forge a slug like that down into a large disk---using a powerhammer and then using air impact tooling hotforge the disk into a large bowl.
Copper is a LOT harder to cast than brass/bronze. Higher temperature and greater sensitivity to O2 pick up. Not as fluid a pour.
Note once it is cut up it will never be as big again---you may want to save or trade it and work small stuff into small stuff.
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Thomas
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