A new case is NOT the answer. Address the cause of the heat. First tell us exactly what model you have. Next tell us about the heat issue. You may just need a fan.
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'''UPDATE'''
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After thinking about your question, my first guess is that it's a MacBook that shipped between October 2009 and April 2011.
A new case is NOT the answer. Address the cause of the heat. First tell us exactly what model you have. Next tell us about the heat issue. You may just need a fan.