I’ve done it! The basics:
Main earth wire is bolted to the engine bay on a clean metal surface (no paint). I left the original terminal in tact and put a longer bolt through the tensioning part. It also acts as a jump start point in the engine bay now too.
Positive side, you need an isolated jumper block in the bay. I used one from a BMW e36 and affixed it to the engine bay around the gearbox mount. Be sure to not have positive wires touching or rubbing the body. All the factory positive wires were terminated to have eyelets and bolted right up to the jumper block.
Had a main positive wire terminated in zero gauge (overkill). Ran this through the firewall grommet and along the sill panel into the boot. Eyelet for the engine bay, terminal ring for the battery/boot. I wouldnt do this again, the alternator charging causes interference with the audio setup. Probably better to run it under the car with conduit to protect it.
earth strap in the boot is a generic short cable. Ring terminal and eyelet. I added a rivnut in the boot for this to bolt to. Again paint removed.
Securing the battery; I stole the original tie down from the engine bay. Drilled two holes and slotted the hooks in. Tighten down battery strap as per normal.
If you go for an AGM battery like me, drill another hole for the vent hose.
