[Steve C]

Hi

If you look closely at the top photo you can see the outer piece is a stock beetle disc with its braking surface removed, then you can see a thin aluminium spacer between the cut-down stock beetle disc and the Porsche rotor.

So to answer your question, I just used high tensile studs screwed into the threads in the cut-down stock beetle disc with a dab of Loctite to hold them in.

Steve

PS, there is a seller on Ebay Australia who sells these high tensile studs called "vforceracing".