Battery in trunk

I'd want the attachment point to have similar thickness as the wire.
Exactly. And as I said, we've all seen rust, corrosion and dirt get between welded seams, especially a single thickness thick. It's just smart to locate the ground on the thickest point possible. I also use 1.0 gauge whenever I relocate to the trunk. In fact, I have 1.0 on my truck with an under hood battery and I plan to put them on Vixen, too.