Is this the only time that your legs and bum go numb - sitting in a kayak? Or do you have some sort of back problem/sciatica nerve problem? If this is the only place, then a custom carved seat that gives you more support all over should be the answer. High density foam that will not change shape or "squish" down will give lots of support where you need it and with four inches deep to work with, you can support more area, go from 1/4 inch thick at thinnest area to four inches at front under your thighs and support sides as well.