Well, I think I'd go for the biggest topper I could find for the automotive battery...
But for an automatic battery... depends. If the stick / 510 style, while anything 510
threaded would go on and work, the bigger toppers would look a bit odd... The Vivi Nano is about the same size and shape as a regular cartomizer, easy to fill, and vapes well. Early models had some leaking issues, but maybe that's cleared up by now.
Cartos, of course.
The Protank mini (or Davide mini) might not be too overwwhelmingly large...
There are also larger autos that can be found. I have a couple of 1300s that work well with Evods, the T3S and similar.
I'd keep coil resistance at the low end, say 1.8 or 2 Ω. With the fixed voltage, you'll get a better
vape than if you go to higher Ω toppers.