Some cushions that I like, are Thermarest “trail seats”. Self inflating, cushy part is 16” X 12”. At first, I would take a fully inflated one, and blow in more air to make them more firm. WRONG! Best practice is to let it fully inflate, then hold a closed fist into it lying on a flat surface to push out some air, and close the valve. This provides a cushier seat of more area, making less PSI on your rump. As long as your butt bones aren’t pressing a solid surface - missing by only 1/16”, you’re sitting on air!
They also make a cushion for lumbar support (nominal 16” X 5”). You can sit on these during the day paddling, and sleep on them at night as pillows.
Pete, I don’t advise sitting on your PFD. Think about that, partner. Do you drive while sitting on your safety belt?