Have you tried one of the donut-shaped cushions? They have the advantage of coming in inflatable models to cut down on the bulk when packing them.
So far I've only tried the one I noted in my first post, but I felt like it was too big to lug around the airports and plus it was a little too thick and not really giving me much releif even when testing it at home in the office, so I returned it.
Something that could deflate or collapse down would be perfect. I'll keep looking, thanks. Maybe something like
this might work.