Thank you very much for the write up. I used to make custom seats for my kayaks. The difference in comfort was monumental. Whereas with stock seats I'd start to lose feeling in my lower body after 15 minutes, with a good custom seat, the only limit would be my bladder. One key that people need to understand is that with a good fit, padding is unnecessary. If the load of your weight and impact is spread out evenly over your lower body and legs, you're good.
Here are instructions that would lend themselves well to car seats as well. Making an insert for an aluminum seat would be perfect.
First make a 4-sided cardboard form that delineates the outer limits of the seat. The bottom is open to the boat, and the cardboard is taped to the hull. Then you line it with 4 mill plastic.
Next you put on a pair of disposable painters coveralls and mix up a big batch of expanding foam and pour it into the mold. Then sit in it in a paddling position and things get interesting
The expanding foam grows around your lower body like something from a science fiction story. It is undergoing a chemical reaction so it's also warm but not hot. 10 minutes later it's encased your lowers and has hardened up. At this point, you look desperately for something to cut your way out of the foam. Fortunately, you're a smart guy and have a knife handy. Cut your way out and try and avoid your peripheral artery.
At this point you'll have a perfect impression of your lowers - hopefully including lumbar and under thighs. You break out a rasp and a sureform and start whittling away till it looks fairly attractive, then apply epoxy and fiberglass. Note: Do not use polyester resin, as it dissolves the foam. Use the good stuff.