I am helping by saying instead of trying to figure out if you just barely make it into a seat go on a diet or go to the gym, start walking now. Instead of making things change for you why not change yourself for the better? I never said I was thin. I think I'm average weight, but that takes work. Before I go on vacation I try to lose a couple of pounds if need be. I don't find modified chairs to be a the right idea in a world were obesity is an epidemic.
I came to this thread to see if i could help. I was trying to be the voice that said, "No it's not ok that your're overweight. You can do better!

You can change so that you don't have to be uncomfortable in an airplane seat or have to worry about getting on a roller coaster.