Great 3/4 inch thick CCF pads for kneeling (perfect for sitting too) are available for very cheap in the gardening section of some retailers. Got a couple recently for just 4 bucks each on a sale. My old sleeping pad, replaced with the Therm-a-rest Luxury a while ago, is safe in the closet for now.
(In fact mine is of a heavily ribbed construction and may not work as well for a sitting pad for several reasons).
By the way, I saw some UL campers attaching a piece of rope or bungee cord to each side of their sitting pad, and wearing it right over the dedicated rear spot all the time. Looks convenient.
Also I know that wise hikers are precutting their pads to 3-4 pieces and linking them back with cloth bands. So they can neatly fold it to a sitting pad shape when necessary without destroying its primary function and making the seat higher at the same time (It seems to me - that idea was commercialized recently, as I saw something folding like that at REI once).
And of course there are plenty of specialized lightweight harnesses available for almost every type of sleeping pad, which converts them to camping chairs. It's not a rocket science to just reverse-engineer one of them and improvise one to a personal taste.