Hey here is a tech how-to on making a seat ,for you steel, doit yourselfers. Its almost the way I do it.I just dont make a mold out of it ,it ends up the same way any way .
Just a tip for the home builders that attempt this :weld from the underside, gives you a smooth out side appearance and saves a lot of grinding.For the perfectionists...it makes it hard to grind down welds.Its why its called custom fabrication .you make it your way.
Any way its cool .here you go. http://home.brisnet.org.au/~jmiller/leinfam/seat.html