Clothes padding

Does anyone have any tips for doing a realistic backfat padding ?

What do you mean exactly? Like adding back fat rolls? If that I never really thought about that but it could be quite challenging to do solo. The only thing coming to mind is a pre padded and shaped shirt that you just pull on. I personally don't bother, I just wrap blankets around my torso so I am padded all the way round not only in front.

yes, I've been thinking about it for a while but it's not easy to do

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.