I've just installed mine next to the ABS on the passenger side. 55AH Optima for size and the fact that I guess I wouldn't be able to have it 100% upright.

It was a bit of a pain to be honest, I used a standard tray from SCA, but this raised it too high so I cut it in half, rewelded about 5mm narrower, mounted with about a 5 degree tilt forwards.

Big support arm on the engine side leading down to the wing, and a piece of aluminium shielding the battery from the exhaust, just in case and based on others.

I had to move the alarm horn (to underneath the power steering/ace reservoir), extend the cable for it, cut off the heated window connector, unwrap the loom to reroute the inertia and abs wiring.

Then an adjustable hold down from SCA, this needed the rods cutting down to clear the bonnet and I had to move the tray across about 2cm towards the engine as they then fouled the abs pipes. I realised afterwards I could use a cargo strap to avoid that issue and may still do so.

The alternative was to have it in the bottom of a bin with a cut out and maybe in a happy removable box. An anderson connector to hook it up.
The only thing is you really have to put some good cable all the way back, in theory the alt can put out 120amp, and I guess the battery could try to draw that, so about 150amp cable to the rear, maybe more to mitigate voltage loss.

I still can't decide if I did the right thing, but I wasn't willing to pay out the price for the ARB tray.

I saw the dual battery solution in the standard position, it was cost, it would have been about $400 for each battery and the tray so about $1000 for the whole job.
As it was I think I have spent about $500 for battery, tray and dual charge system.