I don’t know if this would work, but:
What I’d do is take a cast of the back with Alginate—Alginate is skin-friendly, but doesn’t make stable moulds for more than a few hours.
Reverse cast a plaster version of the back, and while the plaster is still a little soft, make any changes to it that would cause it to straighten the back. Then use the PMC 121–30 WET (linked above), which is a 30A durometer rubber to make a permanent negative mould.
However, if you’re just looking to make something that pushes back against your body, check out Memory Foam—it’s pretty cool stuff.