When I relocated my battery to the trunk I did the following:

1. Positioned the plastic battery box over the unibody frame on the passenger side of the car (gas tank fill side).

2. Fabricated metal frame tray to hold battery in plastic box. Drilled though tray, box and chassis. Bolted down. Drilled another hole outside of box. Bolted battery ground to that one with a metal tab on bottom of car for a jumper cable ground connection.

3. Installed kill switch on back of car and ran positive cable through it and then up to front of car. Positive isolator block mounted to strut tower. Positive cable to starter and positive wires to fuse boxes (I have two).

4. Welded large nut to frame. Ground cable from engine block to frame as well as ground cable from starter to frame.

5. Alternator return charging wires routed directly back to the battery in trunk. (True Kill Switch)

Cables were #2 in size. The setup gives me good ground connections.