They sell a bunch of different kinds of back supports for car seats on Amazon. Maybe u guys could try one of those and see if it helps.

 

Just to verify, you have adjusted the lumbar support on the seat yes? It's shown as flat in the previous picture because the lumbar adjustment handle is in the flat position.

 

it's better, it's not what I like, but it's better. Like one of the posters pointed out, it doesn't curve in again at the top 1/4. The headrest does, but not the seat...That seems to be the biggest issue, at least for me..