I am not an automotive engineer but some thoughts from someone used to high energy power systems.

Assuming that battery is AGM technology it is safe in a rollover, so long as your support means are sufficient to restrain the battery from coming loose. It's one thing to have the battery sitting in a holder in its normal position, quite another to be giving it several g's in a non-normal direction.

I would also ensure the battery cable to the starter (since it's impractical to protect it) is routed with additional layers of jacketing and definitely inside the passenger compartment and ideally along the spine of the vehicle.

Also you should provide some sort of overcurrent protection if you end up tapping anything directly at the battery towards the trunk (wvo hardware??)

If your AGM battery has a vent tube be sure to direct it out of the vehicle.

Also provide an emergency disconnect accessible without removing the seat. My W210 has one on the back of the right rear passenger's foot space. Where there heels would be. There is a plastic cover with a 1/2"/13mm bolt that when removed breaks the ground path to the battery.

If you would like a peek under the seat of my W210 to see how it's done let me know.

They had a couple W210s at the Cola pullapart a few weeks ago but I think they might be gone now. That might be a good thing to do, grab the battery hardware and rear fuse box and emergency battery disconnect hardware.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech