> . . . The idea of containing the mold is to make it light and thin - say 1/2 -
> 1" thick. So that you can move it around , cut it and shape it
> easier. Or add more plaster etc. here and there to fix it, say extra
> burlap on the back.
> . . . take an old pair of
> jeans, cut off most of the legs, smush them thru a bowl of plaster and
> pull them one and sit down on some foam or any thing that's soft!( no! not
> the cat!) As they're setting up, cut from the waist down the tops of each
> leg so you can get out. Maybe a real light form? jeesh, maybe I should
> think this one through!

> . . . What you really
> need is something like a thin rubber sheet that will push down and not
> wrinkle when you sit in it to make a smooth form . . .

The idea that the plaster of paris needs some support is certainly valid. Hobby, craft and art supply stores already sell rolls of a cheesecloth-like fabric which is impregnated with plaster of paris. You soak the roll in a bucket of water, unwind as much as you need and lwt it harden.

This is very much like the material that doctors used for years to make casts for broken limbs. It gets warm as it sets up, but it is not oncomfortable.

If you have a seat that is close to what you like, lay maybe 10 to 15 layers of this material on that seat, and then sit on it while the plaster hardens. You should get a thin, strong shell that is curved on the inside to resemble your posterior. Trim this carefully, and you can build up sides with more of the plaster-filled cloth material to make a mold for your fiberglass.

If you can't find this material locally, try Dick Blick art supplies you can get a complete catalog by mail from their website. (www.dickblick.com) I didnt see it offered online.

While I was looking, I found that Dick Blick has another product that might be the most suitable item around. It is a moldable wire mesh. I have not tried it, but it seems to me that if you would just lay this on your foam seat,or a pile of sand, or even set it on your sofa cushion in your living room -- and then sit on it. The wire mesh deforms to match your butt -- and holds that shape when you stand up. It is dry, not messy and fast. You can cover it with fiberglass and incorporate it into your seat, or cover it with a thin layer of saran wrap and pour in some plaster. It is an even thickness, so you can make a male mold from one side, or flip it over to make a female mold. Here is the URL to their catalog page with this product. A roll of the material is about $16 and should be usable for a variety of things where you want to match certain contours.

http://www.catalogcity.com/viewprod.cfm?pcd=178125&isz=m&plt=1

Hope this helps

Paul G. Jacobson