When I went to 17" wheels, I had to get a used spare tire with rim. My initial solution was to place the tire in the wheel well face down so I could secure it with the standard Audi plastic lock. But, I had to fill the inside of the wheel up with wood to support the plastic wheel cover. The new solution can be seen in the 9" long rod below the stock Audi plastic lock:



I cut an M8 1.25mm 36" rod down to 9 3/8" length and put two nuts on one end that would fit into the Audi plastic lock:



Then, I set the spare wheel and tire in the well face up:



Next, I secure the whole thing with the 9" new rod. In this picture, you can see the Audi plastic lock is in place with the rod secure into the threaded piece in the bottom of the spare tire well:



Finally, to protect my spare wheel, I cover it with this blue towel and I neatly shove the ends of the towell down beside the tire to give a smooth surface to accept the plastic wheel cover you will see in the next picture:



Plastic cover:



It almost fits perfectly. It is raised by about 1" at the lower part of the picture, but close enough to work.