I don't often get numb legs, but my problem is in my back, despite several physiotherapy sessions over the years. My kayaks both have nicely shaped seats but I have to straighten out every half-hour or so or I can hardly walk when I get out.

Both have a bump in the seat between the thighs that nails me right in the tailbone when I slide my butt forward.

On my home-made boat I built a handsome slatted wooden sliding seat with a higher than usual back that hinges back as the seat slides forward. This allows me to relax and almost straighten my back periodically, and solves the problem. With head support, I could sleep in it. Despite being a hard seat it does not cause numbness, probably because I curved the bottom and back to fit my body, so pressure is distributed.

The downside was, it took almost as long to make as the hull, but it gets a lot of compliments. Unfortunately, it takes up too much space to be implemented in a typical kayak.