Hi I was asked to provide some details of the rear spare wheel mount on my S3 See below I hope this is helpful.

This image shows the bottom plate which is 250 by 70 mm. Uprights are in 50 by 25mm steel box section. Bolts to suit through main rear cross member.

Rear of alloy number plate mount size to suit

Bracket for numberplate mount. Bracket off set to suit spare wheel.

Main wheel mounting bracket. Uprights 210mm, horizontal 150mm.

Similar to above

Rear view showing steel angle (40mm) wheel mounting square (150mm square) that is welded onto horizontal support. Two verticle hole have captive nuts to bolt wheel to. Two horizontal studs pass through wheel to mount number plate bracket to.
Wire for number plate light!
Barry