For my bigger malts, I use their normal harmness vests. They sit in a large "outlook" car seat and I harness them in by putting their stepin harnesses on and using a coupler which I tether to the head rest. If you need a picture, let me kmow. It works good.
For Ava, my tiny girl....I put her in a small hard crate in the back seat straped in by the seat belt.
You can never be too safe.