Yann, as someone posted further up this thread, you can get a mounting that fits to the luggage rack to take a spare wheel.
That is your easiest route. Otherwise you are looking at major bodywork/frame surgery to the rear of your car...


Graham (G4FUJ)

Sold L44FOR 4/4 Giallo Fly
'11 MINI Countryman Cooper D All4
'90 LR 90 SW