12-16 hour flights in economy are hard on my body as well. I spent $$ on "First Class Sleeper" and it was so awful I abandoned it on the plane.
Inflatables are easy to pack, but I cannot imagine sitting on one, I would wobble constantly. I haven't tried this one but I like the basic idea:
http://www.magellans.com/store/Healt...&istobe-type=0
Bigger to carry, of course, but since in my case the pain develops around the tailbone, maybe this will help. YMMV.