A closed-cell foam sit pad works for me. Cut from an old sleeping pad, or you can buy them. Also goes under my feet at night. I've sometimes carried a closed-cell pad for sleeping, and while not the most comfortable for that, it makes a great chair for lunch breaks and general loafing. The 3/4 size is long enough that you can sit on one end and use the rest of the pad for a back rest, propped up against a rock or whatever's handy.
Always remember that you are absolutely unique, just like everybody else. -Margaret Mead