Hello,

You just now need to make a sweep using profiles from the part you have made(with the rads), using rails from the seat part.

Select the back and join the edges for the profile, then select the seat, and copy paste the edges for the rails. Then use sweep.



Just be careful with the rails, you will need to trim/rotate the back rail or you will get a bad blend (the surface will have creases)



Hope that helps, if not, I will post the model and explain more.