in the past, i've used some cardboard, even a 'cut out' of the shape/angle of the fade...hold it up to the body, shoot AT the cardboard and that will give ya a pretty decent fade line...
...and any new car dealers close?? they, well, many, use a foam stick on pad[about 3"x1"x1"+-]on the doors during transport, i use them for sanding blocks as they will conform to ANY body curve...