My car has a spare wheel. I disagree with its carrying position, which is clamped to the left inner rear wheel arch. That is because it prevents the seat from being able to go fully back. It could fit across the rear of the inner on the back body panel, but in my car there is a battery tray in the way. This takes a larger battery. Remember it had naff fitted dynostarter, so I suspect rather than fix a botch, the extra botch was a bigger battery. Doh!
I think the original idea was that one cubby hole pannier on the rear inner wing took the bike sized battery and the other the tool kit, the Gitane and a spare bottle of wine. With the rear empty of gubbins it is possible to have a small passenger, dog or shopping in the car without hassles. You tube clips show the cars used thus.
So the wheel could be fitted on the rear, resting on the bumper, clamped onto the body. Best with a matching cover to the seats, green in my case. But my preference would be the void under the rear floor as it gets it out of sight, gives a lower centre of gravity and keeps the interior empty. It would not be hard to find a bin to then bolt over it to keep it clean, with four drain holes in the bottom.
Now suffering a poor leg as a result of the contortions I had to do on my knees to get at the restricted light fasteners. Just proves I really have to back out of this as a not only a business, but as a main hobby. The Doctors are winning the argument.