Bought this one due to having high hamstring tendinopathy (couldnt sit in my regular seat without feeling excruciating pain in my ass/hamstring):
http://www.amazon.com/Yakpads-Paddle...eywords=yakpad
Worked really well for me. I like the 3 loops it has... makes it easy to strap down to the seat so it won't slide around. Plus, I think there is some grippy material on the bottom to further prevent it from shifting around.