A metal bowl over a pot of hot (not simmering) water will do, but having a true double boiler insert will go a long way toward make stirring a one handed job. You also don't have to worry about the bowl tipping or swiveling during melting and especially after you remove the bowl from the pan. Some people use a microwave to melt chocolate in short bursts, but I just don't believe you can ensure that some of the chocolate won't get scorched this way. Even in bulk, chocolate is expensive—treat it accordingly.
