I've found that the best seat cushion is actually just a memory foam neck pillow like this one. On long hauls when I'm not using it for a neck pillow I will put it on the seat as an added cushion.

The trick is to turn it around so the open part is in the back. That will take the weight off your tailbone and make you a lot more comfortable. It will also put more weight on your thighs, which is a good thing. By distributing your weight more evenly up your thighs you will reduce pressure/pinch points that hurt.