Audi R8 Headlight installation help on 1999 Audi A6 Quattro

I have a set of Audi R8 headlights that I would like to put on my A6. They are just barely too small leaving a slight gap between the bumper and turn signal. I don't want them to bounce around as I am driving therefore would like to know what the best material is that I should use to mount them so that they fit snug?

Thank you for your help!

Asked by for the 1999 Audi A6 Quattro