If you are seeking to make a mold for a more durable fiberglass seat you can certainly use the foam. Put the foam in a medium sized garbage bag (7 to 13 gallon size should work), let it expand a bit, place the bag in the kayak and sit down on it before the foam hardens. A pin hole in the plastic bag lets excess air out, but is too small to leat the foam ooze through. Your body will press out most fo the folds in the plastic, and supply the shaping for the seat.

Check out the price of your foam. It may be more economical to use other materials for making a mold. the process is the same -- put the molding material in a plastic bag, position it in the kayak, and sit on it -- but the molding medium is different. A small bag of redi-mixed concrete is only a few dollars. It is heavy (you'll use about 10 to 30 pounds of the stuff) but sets up fairly quickly, can be mixed in the same bag you'll use as a mold (don't add too much water) and the amount can be easily adjusted, just start with a small amount and add a bit more if needed. with a stiff mix you can make the mold, carefully climb out, slit open the top of the plastic bag, and smooth the surface of the concrete. A stiff mix, made with less water than you think you'll need) will not run all over the place and make a mess. Then leave it alone for a few hour whil it sets up. If you are in a hurry, you can use plaster of Paris in the same manner, but you should plan on working quickly with that.

When the molds are made you just cover them with resin and whatever scraps of glass cloth you have left over from your boat. You really don't need to make a female mold. A single layer of glass cloth is not going to make the seat size significantly smaller. Remove this from your mold when the resin is cured, but green, and add additional glass cloth and resin to the inside, building it up to as thick or as strong as you feel you need. You can insert wood or metal pieces for reinforcement, too.

Hope this helps.

PGJ