You can get an 80 pound bag of premixed concrete for under 5 dollars. Put a layer of newspapers on a flat surface. Place a frame made from 2x4s on the papers and add a stiff (not watery) mix of concrete. Use a board to get a flat surface. Use about 3/4 of your bag of concrete.

Cover the concrete with a plastic garbage bag or similar thin plastic and take a seat on top of it. Wriggle around a bit to get a good impression the get out of it without disturbing the plastic. Let it harden for a day. After that you can remove the plastic and let the concrete dry, or leave the plastic in and it will serve as a mold release. If you remove the plastic you'll need to seal or oil the mold to keep castings from sticking. If you are planning on using tthis a lot, let it dry completely and paint it with a glossy enamel or varnish which you can wax and oil.

Mix the remaining 1/4 bag of concrete and pour it over your mold to make a plug which you can also use to cast your seat. Seal the rough plug with two coats of epoxy then clean it up with sanding and a third coat of epoxy to get a slick surface. If you want a textured surface don't spend so much effort on it.

The outer surface of your seat should be glass cloth. If you use any carbon fiber cloth keep it under th glass. Scraps of glass or carbon fiber cloth may fit the contours of your seat better than larger flat sheets.

Minutes after you've put the glass into your mold prepare some flat rectangles for the seat sides. Wet out 3 or 4 pieces of carbon or glass and stah them on a flat surface.

Let this rest for 12 to 24 hours. When you remove it from the mold the parts will be 'green'--flexible and easy to trim. Trimm off any edges from the seat shape, cut sides from the flat pieces and glue them together with epoxy_saturated strips of glass cloth. When the thing gets hard sand away the rough edges.

After you mold the shape of the seat you can add more fabric and resin to increase the strength and stiffness.

Hope this helps

PGJ