Steel is not practical for a DIY'er. It will be too heavy, and will require stong attachment points, which are probably not available.
If your design involves gores or sections of streight planes, as apparently it does, then why not just use half inch blue foam to make the gores and glue them together at the edges. You can then fiberglass the entire structure, inside and outside for a strong and light structure. You can make your attachment points from wood which you will glue into place and cover with fiberglass, making them a part of the overall structure.
See this link for a very complete development of a boat tail for a first generation Insight:
http://ecomodder.com/forum/showthrea...ect-13533.html