Sheila, I filled a garbage can with the waste from making one but here goes:
I made mine using the cockpit for the mold so that the seat and under/side of thighs was integral with the hipbraces that also served as the hanging attachemnt from the coaming. A dam was made about mid thigh in the floor of the kayak, another a few inches behind where the back of the seat ends. This is the cheesy part so I'm up for suggestions, using a couple big garbage bags one inside the other lay them into the mold area. Mix up about 6-8oz foam, pour it into the back of the bag, sit (I wore a wetsuit and two pair of sweat pants) on it so with opening gathered between your legs. The foam will not fill to shape in one application and it's necessary to lift yourself up by your elbows for approx 30-60seconds after the sides and back are full so the foam can mold to ones toocus other wise the mold will be of a butt squished flat on the bottom of the kayak. The problem with doing it this way is that the bag has to be opened up a few times after the foam hardens without tearing the bag, also the bag may not be big enough to conform at the sides. I made one mold with a couple of kitchen sized bags on either side. After getting the last amount of foam into the bag,,sitting in place, extricate yourself from the seat and go at it rasping, cutting, shaving it so that it's comfortable. A true mold would lock you into place with no room for movement, especially where the foam rises in the front. Round over the front and back of the seat, make sure the mold is low enough in back so that a back band can fit ok, for me that was a seat about 2.5" high in the back with hip/hangers mold continuing to the coaming. If that wasn't fun enough theres more. With mold smoothed out tape another plastic bag over it. I used a big piece of newpaper to help determine the shape of the glass to be layed in the mold, the idea being that the glass will cover the seat area and come up the sides over the coaming. That way when you sit back in the mold with all the glass underneath you the sides of the seat can be taped up out of the way onto the deck (masking everywhere). The cloth I used was a knitted biaxial, two layers of 12oz I think. I'm running out of steam describing the process but you get the idea, with polyester resin it sets up quickly,,,mask everything,,,,,lots more cutting/finishing. With pigmented epoxy finish it looks quite snazzy, made one with two layers of 5oz carbon/kevlar,,way snazzy. I think the sand mold method would be a lot easier but having the seat mold up into a thigh mold is a big part of the comfort, nobody has straight hips when sitting.