wisdom needed please regarding wheel centre caps please

[s4marsh]

hi all,i'm wanting to fit rs4/tt style centrecaps to a set of b6 18"avus wheels like i've seen in this picture but think i'll have to modify them with the original ones, some of them are the right size 150mm face but others are 147mm and i need a 57mm centre on the back but none of them look deep enough....i'll post a pic to show what i'm wanting to do i'm guessing i'm gonna have to modify the rs4 ones with the back of my s4avus ones???? or is there a genuine item that is a factory fit?