I have been using this REI Sit Pad for a couple of years.
http://www.rei.com/product/845302/rei-sit-pad
It deflates and rolls up to water bottle size. I even use it on 3-4 hour domestic flights and won't travel without it. REI has a 1 year no-questions-asked refund policy and a lifetime warranty for defects on their products.
I've also started using it for long car trips and even for....hiking/camping - its intended use!