I happen to be the proud owner of about 10 seat cushions, since the onset of my bad AF on Dec 22! Some have helped, most haven't. My 'donut' is getting 'donated' soon (OK, pun intended). It never made it past a very short sit. The best one I have found for home or office sitting was provided by a Guest at this forum. The link is:
http://icnshop.com/cgi-bin/ustorekeeper.pl?command=goto&file=DailyComfort-ChairCushions.htmlIt's the one with foam under both legs but nothing down the middle. I love mine! They have them for different weights as well.
I just ordered an inflatable cushion to take on a flight next week, but haven't tried it yet. I also use a dense foam cushion I bought a while ago at Office Depot in my car.
I hate sitting, because it strains my fissure, but sometimes you just have to! I hope you can learn from my mistakes. The other one I didn't like was made for people with tailbone issues (a cut-out hole towards the back). Perhaps others have different experiences? :)