stovepipe heat shields-- specific requirements?

Are there any particular requirements for stove-pipe heat shields? As far as I can tell, the commercially available ones are just a rounded piece of 24 gauge steel, but are not exactly sized to specific uses—i.e. are for 6, 7 or 8 inch pipe. Is there any reason I cannot simply use a half- piece of 6 inch stovepipe off set one inch? Are there other specific requirements for stovepipe heat shields to meet NFPA guidelines, etc?

thanks