It is tight under the hood!
I have seen other people on this thread put a pair of Optima batteries sideways (90 degrees from factory placement) I think it was. Similar to how I have two in my 04 Discovery. (yellow and red, with red being more a starter while yellow is better for deep draw and running a fridge, etc, supposedly) There are different sizes of those though. This brand in particular has big rounded corners and a top to bottom taper which may be the characteristic allowing fitment of 2 in your factory compartment.
I have an Odessey 2150 though in my lr3 which is provided with a 2nd battery slot yet we had to customize the space to fit the much larger 2150. Their website has a really good matrix of sizes and outputs you might find useful.