Making a mold for a kayak seat is as simple as adding water and sitting down.
For a mold material you can use the expanding foams, but they are rather expensive, and get uncomfortably warm as they set up. There are cheaper alternatives. The one I suggest most often is what I call the Chicago Swimsuit: Concrete. A bag of Sackrete sand mix is about $3 at your local hardware homecenter. A plastic garbage bag is a few pennies.
Place the open garbage bag on the ground, mix up a fairly dry batch of sand mix concrete (don't add too much water, just enough to barely dampen the mix) and dump about 30 pounds of it into the plastic bag. Remove the air from the bag and use a metal twister to close the bag. Then sit on the bag. Wriggle around a bit, and stand up.
Now leave the bag alone for a day or two while the concrete hardens, giving you a perfect impression of your butt, and providing a fine base for laying up a plastic seat which matches your anatomy better than any store-bought seat ever could.
If you want a mold which also matches the bottom lines of your kayak, just set the boat on a firm surface, put the plastic bag in the boat, add about 20 pounds of sand mix to the bag, and sit down for a few seconds.
Let the concrete mold harden and then cure. Scraps of fiberglass cloth or tape saturated with resin can then be applied directly to the bag, or for a finer finish, remove concrete from the bag and fair?.
If there are impressions in the concrete's surface from the bag, or air voids in the concrete, these can be filled with plaster of paris and later sanded. The plaster/concrete cast should get a coat of polyurethane or other sealer, then waxed. Johnson's paste wax is fine. PVA mold release or thinned white Elmer's glue can then be brushed on. The wax keeps the mold release from sticking to the concrete, and the mold release keeps the epoxy from sticking to the wax. (The mold can alternately be coated with mold release, then waxed)
Vacuum bagging would be a great process for this type of construction. You could insure that all of the glass is evenly pressed against the mold, and also minimize resin used for a lighter, stronger seat.
Apply the cloth is 2 to 4 layers thick. More glass strips can be added at the edges, or in areas you might bolt through into the boat. Let that harden for a day and remove it from the concrete. Now clean the back of this and try placing it in your boat. Trim any overhanging glass and resin while it is still "green". If you overtrim, don't worry.
Add more glass cloth scraps to the back until you are happy with the strength of the seat. At this time you can also fix any overtrims. If you want, you can stuff foam rubber or minicel under the seat, too.
A few fill coats on the surface to conceal the weave of the cloth and you are done.
It goes faster than it sounds, and it is easier than you think.
I made this mold by sitting in a box of wet sand in my wetsuit, then covering the impression with burlap and plaster. This mold was then painted, covered with an acrylic form coat (made for concrete forms, but it sanded really well), repainted, and then waxed.
If you use "Post-Haste", a premix product designed for fence post setting, which sets while you hold the post, you will preserve your posterior promptly. It can get a little warm but before you bake your buns it will have set enough to bail out.