I fully agree that an airline seat is more comfortable than bleachers but how do you put your bag under your seat - there's almost always someone else's feet or bag there?
In that scenario, I put the bag on the tray, bear hug it, and drop right off. Leaning forward into the bag works for me.
(Yes, I have a huggable bag, and I have one of those thin blankets I mooched from Delta years ago.)