Maybe this is easier to veiw.

Use 1" foam around backrest (medium density). Cap the top with landau top foam 1" thick to make the curve on the top. Make cushion. Cut foam at height just below top of cushion. You can pattern the shape of the backrest and use snaps to secure like in the photo. If you wanted to make a neater finish you could go with a wood perimeter but you will need reinforcing and a complex amount of time and energy to figure the geometry.
That is going to be a very tight inside radius wrinkling will be a big issue. I have found shrinking the pattern and over exaggerating the curve to put tension on the apex is helpful.
These are difficult seats to make due to all the small inside radius. Trust the patterns.