How did you miss the giant picture of the rule book??? LOL
Battery CAN NOT be in the cabin, at all.
Your foam back seat is NOT a firewall. Behind those seats its not a continuous, uninterrupted sheet of steel/aluminum, aka a firewall. So it needs to be in a box. Also its not just a “drop it in a box and go” deal. The battery box has to be bolted down. and the battery in it has to be bolted to the box.
Lastly this “sealed” battery stuff has nothing to do with any of this. Sealed, serviceable, gell cell, or a massive pack of RC car batteries could be used… none of that has anything to do with the rules.