I used to paddle a Chesapeake17 with a happy bottom seat (still do on accasion) and experienced the sore leg/foot numbness problem. Somebody gave me the tip of placing one of those al cheapo foam sleeping mats (still rolled) just in front of the seat so that it provides a bit of support under the legs just in front of the bum providing support much like the seat illustrated at start of thread. It worked very well.
I now paddle ocean/surf skis which have a hump in the cockpit between seat and footwell and i have not experienced leg pain/foot numbness since. I'm certain that little bit extra of support at the top of the hammies/bottom of ass makes the world of difference.
Cheers