Looking to make a composite (carbon/fiberglass) seat for my kayak and was wondering what methods you would suggest to get the right fit.

One thing that comes to mind is to make a mould taken from my own behind sitting into something that would retain my butt's shape after I get up (and hopefully won't hurt me in the process -;) )

I saw this http://www.kayakforum.com/cgi-bin/Building/Archive50.pl/noframes/read/53500. Also remember reading about using ready-made plaster sheets that can be formed as desired, but lost the link and can't seem to find it - can anyone refresh my memory?

Basically, looking to make a slippery lightweight composite seat with a raised back just enough to keep me in it when it gets a bit bumpy. The seat will be mounted to replace a factory seat in my boat. I have a foam seat already and that works fine but I want something that would allow me to move about more freely, sort of like in racing kayaks and surf skis.

Suggestions on shaping it and construction hints will be most appreciated. For instance, what would be the overall thickness I would want to get to to ensure it won't crack too easy, etc. I will probably be able to do a rudimentary vacuum bag with my food saver so looking to make that in one shot if possible and to make it lighter and stronger.

Thanks!