'risk of fire' ?? I don't think so.
The current battery compartment exterior should obviously be very strong metal, capable of withstanding blow-out damage and road debris. If it isn't, it will need to be replaced. Yiou installer seems to be confusing BattleBorn LiFePO4 batteries with batteries using other kinds of Lithium-Ion chemistries.
LFP batteries don't have the high fire risk of other types, and the excellent internal BMS installed by BattleBorn prevents overcharrging, overcurrent, and other "fire-provoking abuse" in the first place.
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Within a strong aluminum box, my only concerns would be theft and likely low outside temperatures during mornings. BattleBorn batteries like to be inside, because they can't be charged at a high rate when the temprature approaches freezing (or falls below freezing in which case they can't be discharged very well either).
Some of these LFP batteries include built-in heaters, which warm them up before trying to operate in cold temperatures. I don't know if BattleBorn offers that feature in any of their models, but it might be worth looking into.