Therm-a-Rest Pro Lite seat is the one I purchased. It has rounded corners, is very lighweight, rolls up smaller than a can of soda, and holds up to my extremely zaftig rump. At about $25. it isn't cheap, but wow does it make a difference in a bad seat! Comes in a hard-to-leave-behind red, a purple, and two kinds of camo. http://cascadedesigns.com/therm-a-re...e-seat/product

Other threads have mentioned the cheaper REI-brand inflatable seat cushion. Well, reviews on REI's own site are bad at best. It's made in another country and with less care, it seems, than the Made-In-USA Cascade Designs Therm-a-Rest. Don't buy the cheaper one just to save money - you'll just get aggravated.

The seat cushion also makes bleachers and rocks comfortable as a feather bed to sit upon for long periods. Next parade, game, or theme park you go to, take yours along for the day!

Note: it doesn't come with a case. It fits into a fabric 'can cooler' or Japanese water bottle sleeve, or even a mini umbrella cover salvaged from your old worn-out turned-inside-out brolly.