I had back problems ( for years) and surgery for it several years ago. I see you are doing all the things you should do to strenghten your back. That's great.

I use a SUMMIT VIPER XLS that came with a strap on seat and back pad. It works great for me.
If I recall Big Buck Branagan (sp) had his wife make up a seat for him because of a bad back. He may be a resource for you.
My back is far from perfect but Im day to day pain free. Just some nerve problems from the damage that effect my legs.
I can spend all day, sunrise to sunset in my stand and it's more than likely that my butt will get sore before my back does with my Viper