I would suggest looking at the Rorty Design seats, they seem to be a bit more refined that the book ones. The drawing is available for free at the Rorty web site: http://www.rorty-design.com/files/Ro...%20Pattern.pdf
Rorty specifies 2.5 mm 5052 or 5251 ally, which should be equivalent to about 1.2 mm mild steel. Could any engineers confirm or correct?
I found on the internets that substituting ally for steel sheet reduced the weight by a factor of 2, which means that the seats have to use 1.6 mm sheet and will weigh about 9 kg each. flyerncle, you were spot on!