Dual battery system?

My old Surf had twin batteries. Is there room on a 2001 Y plate, LWB SE+ Terrano to fit a second battery? If so, how then to connect? I have a brand new spare battery as I bought it for my works van (Renault Trafic) as I thought the original had died. New battery later and I still had a couple of flat no starts. Checked for parasitic drain down and found that a DVR crash for cash camera I bought last year was draining it. Disconnected it, but wanted to get the new battery checked, came back faulty, so it was replaced. Thing is that I have charged the old original battery and put it back and it's going great :clap

So I have a brand spanking new battery doing nothing. Even if I could fit it on the Terrano and wire in a trickle charge (like a caravan battery), it will be there to jump if needed.

Normally second batteries are fitted behind the ns headlamp. However on later models and I'm not sure at the point this was done Nissan started fitting washer bottles in that location.
There's been a few threads about this but generally a standard split charge relay is fitted. What ever you do fit fuses and also be aware your insurance company May class a second battery as a mod.