I've been using this for a year or so now, it's saved my butt many times (pun intended):

https://www.rei.com/product/845302/rei-sit-pad

It's self-inflating and deflates quickly with a twist-lock valve, not one of those annoying pinch valves. Also because of the ease of deflating you can adjust it pretty quickly to your needs.

20 bucks and it works very well.