The SeatCushion by Tempur-Pedic™
Product Review (submitted on December 18, 2011):
I have an older 4th identical tempurpedic cushion that is perfect. I was told by the store that these 3 newer ones "just needed time to soften up." Well after a year of my husband trying to use one to get it to soften, it is as hard as ever and so are the 2 unused ones. An hour sitting in them sets off my sciatica. The older soft one lets me sit happily for hours.
(I too am around a hundred pounds and live in a cool house but that has not negatively affected the one that works beautifully.)