Liza, I had 3 bowel operations in the past and each time after the op, the hospital give me a rubber donut which you have to blow up yourself, it was impossible to sit without this rubber donut thing, it was approx at least 20 inches in diameter, worked the best when it was only blown up to 3/4 compacity, if you can still get them i am sure they would be excactly what your after, perhaps a pharmacy may stock them.
Brian