how much mm can be widened?

Hello dear Elch community, I need a few infos again, maybe one or the other one has something for me. I have on my V50 215/457ZR17 and behind me the tires sit too deep in the box and I don’t like that how that looks. Now I wanted to know how much mm can I make in without having to make troubles? Bördeln is absolutely out of the question for me. Maybe someone already has track spreaders inside??? If so where can I get them in the www at what price??? If I find Google I’m sure I want to buy something but if there where someone can recommend. In front have right hearty thanks gruss Attila