The Chairless Chair, an invisible chair that you can wear
This appears to be something that's solved with the Asian Squat. It's not fashionable in the West to do this so we don't build up the right flexibility and tendon strength to do it comfortably. But if you can train yourself to do it, you can do it for hours. It's basically human's default "sitting" position.http://www.mykoreanhusband.com/asian-squatting/