Group 31's would have been a great idea if I didn't already have two expensive 34s from my old truck. I think you could rig up a way to keep them secure, but I don't know that it's worth it.
The space is so tight in the engine compartment, I'm worried about the wires rubbing through their shielding and shorting. Or a battery shifting and crushing the OEM wiring to the fuse box. Even if I kept the dual battery tray, I'd need to buy a new primary battery.
I ordered a new OEM battery tray for $70. And $30 worth of Vacuum Solenoid clips. I broke all the originals. Making this a $100 mistake so far. I'm planning to run just the Die Hard Platinum 34 in the Jeep. And I'll stick the new OEM battery in my wife's car. Her current battery is 3-years old.
I'd feel better having a dual battery system, but I think the complications may not be worth it. Toward the end of owning my Land Cruiser, I had constant problems with my dual battery system. They all turned out to be lose wire lugs. But it's frustrating having your fridge, lights, HAM radio and air compressor connected to a non charging battery. Maybe there is something to be said for simplicity.
I'm selling my new T-max kit on the classifieds if anyone wants it. If I end up deciding I need a dual battery system, I'll probably spend the many extra dollars for the Genesis. Maybe when I get a winch.