I recently relocated my battery into the boot of my car. Used 2 gauge cable, ran both the positive and negative cables under the car (wrapped in 2 cable conduit tubes). Ran along the chassis rails near the brake lines, making sure i tucked the cnduit under as close as possible to the body, cable-tie-ing every 10cm or so.
bought an approved battery box from peps ( repco, supercheap etc etc will have them) and finished it off by using som gold plated terminal mounts.