Here's my setup. I did Aza's too, both very similar. You dont need to extend the negative, only the pos. For the neg, just attach it to the car somewhere in the boot. I went to the bolts thats hold the rear strut brace in.
Also keep in mind the legalities, the battery has to be a selaed unit as the boot is not ventilated, you also have to have the fusible link as close the battery as possible, this is just in case the pos lead earths against the car. You also need to buy the proper gauge lead too. I can't remember what gauge it is thou.. the battery shop I bought it all from told me.
You'll see here I used the old battery tray from the engine bay. I had to cut a corner off it to fit & a little metal manipulation as the battery needs to be solid mounted. I also drilled through both the tray and the plastic tool tray it sat in & bolted it solid to the underside of the car. It was a good way to both reuse an existing part & also keep the plastic tray in the boot - as it also acts as 1 half of the boot board support
Was able to hook up the existing battery support bracket as well.
Connectors & fusable link before being connected.
Finished product
