Many people have such problems in marathon racing. Most of the guys that do that pay a lot of attention to their seats...sometimes spending years getting one right. Most of the time they do. You have to experiment with tilt...whatever is comfortable for you...shape and firmness. A mold to your butt is only a start. Sometimes, due to physical frailty, or injury, you'll have to add support in one area...or strategically remove support to lessen pressure in one area. Another area to work on sometimes is your physical condition. A lot of people, I'm one of them, are stronger on one side than the other. This can lead to problems. One needs to strengthen the body equally on each side. The lower back needs to be strong, but so does every other part of the body. Tensing up while paddling can cause problems. You need to stay loose, allow your hips to move. I think it's good that you are working on your seat. Make small changes and try them out. Don't forget to work on yourself. And...finally...there are quite a few other things, medical, etc. that you may never be able to correct, or correct without outside help, or surgery. My advice is worth only what you paid for it. I hope you get this worked out enough to enjoy your paddling. Have fun.