I went with the Happy bottom seat from Pygmy for my Tern....
i carved out underside of the seat(not finished top side) to create 2 pockets
where the butt bones apply most pressure.My leg will still get numb occasionaly but not nearly as much as before....i noticed this happens most often when i forget to use my legs when paddling.The built in hip braces are nice too.
good luck
pete