I've been using brass, but copper would probably be better.

Making a convertible... well, it depends, I suppose, on how much you care to fabricate yourself, as opposed to obtaining from a convertible donor car. Other than the obvious ones, there are several differences, such as trunk hinge supports, windshield frame, reinforced rockers, and reinforced inner structure that holds the top assembly. The top well opening has a gutter/structural system which collects and drains off water. Then there are all the convertible-specific molding and interior pieces.

As far as I know, Ken Hopperdietzel does the best conversions.