Layering a towel flat, underneath a compression shirt / shapewear might be good way to add bulk to your back/shoulders. Though that'd just be a flat layer of bulk, not exactly "back-fat", but it'd be better than nothing. Now I want to try that out...
Could try stuffing socks or some other kind of tube, maybe (with polyfil, or even other socks)? Then positioning those within the compression shirt/shapewear to simulate rolls? It might be hard to keep them in place if you're moving around, but I guess you can't know till you try.
3 days