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