Just to add something different to the suggestions: you could purchase two seats (or a row of seats) and only use one if you are in coach, or even in first.

I realize that this is expensive, and could be prohibitively expensive.

Another suggestion is to bring a hat or cap that you can lower over your eyes to allow you to relax or sleep.

After lots of miles in every seat imaginable, my mild issues have subsided.

I wish you the best!