Wrong priorities. Scope must fit in car. Not on top. Either buy a bigger car or a smaller scope.
It is quite possible to build a 12” that will fit in a small car - back in the 1990’s I had a 12” which fitted the boot of a Honda Civic hatchback. But not a commercial 12”, no way.
See for example a 10” that fits in a backpack (!)
https://www.cloudynights.com/topic/5...act-truss-dob/
You could cannibalise the parts and build your own, which would be cheaper than buying a larger car.