Coming back to this old thread and how I mounted my favorite NMO mag mount 3/8 wave antenna to my aluminum truck.
I used an adhesive backed steel disc/plate from Rugged Radio. About $25. Works great, SWR is OK. A tree limb has already knocked the antenna over with no damage to either antenna, mount or truck.
The downside is the disc is too big to fit between some of the raised ridges on the roof, so the antenna is offset to one side. So I guess there's room for another one to balance things out a bit. As the antenna is black most people don't even notice it.