Hey Banjo.
I dont have a solar setup, but i do have a dual battery system in my gs that could accomodate one in the future. I managed to fit a 120ah deep cycle in with the original battery, but its a very tight fit. Had to temporarily move the side panel as you can see. I had to make up a tray to keep them level, there's lots of info from defender owners available on this, but here's a picture. I then run a bcdc charger, and a secondary fuse box for the fridge, invertor, lights and compressor (see pictures). So far it's all worked ok, but the real test comes tomorrow when i leave for Lorella Springs.
Cheers, Matt.