Thanks very much for your thoughts. I would happily give up the idea of building a steel box if it won't provide enough, or any, added safety. That said, I have to believe that a battery fire could be contained in a steel box if it was heavy enough but that it's not worth the expense and weight for EV manufacturers to do that.For what it's worth, if the modules catch on fire, you're not going to stop the fire from spreading regardless. When a factory EV catches on fire, it's often just allowed to burn out on its own since lithium batteries supply their own oxygen which makes extinguishing them exceptionally difficult. The battery box vent just prevents the battery box from becoming a pressure cooker and possibly exploding in the event of a fire (not ideal!). So whether the box is steel, aluminum, plastic, or otherwise, you're in for a very bad time if the modules catch.
An ounce of prevention is worth a pound of cure. There are only two ways which will cause the batteries to light: physical damage to the cells (eg. a puncture), and overcharging above 4.2v. Lithium batteries can also thermal runaway above about 70 C, but I have a hard time seeing a scenario in which your modules would get anywhere near that threshold given the discharge and charge rate you mentioned.
With all that being said, you certainly don't have to stick to steel. Many folks have used aluminum without issue.
What are you using for BMS/charger/fusing/contactors etc.?
Probably the safest thing would be to use LiFePO4 batteries. They are heavy and stupidly expensive though.
I have questions about what BMS, charger, contactors, fuses and switches I need at the batteries. So far my plan is to use an Orion BMS, a fuse on the negative cable of each battery box, and maybe a power cut off on each box? The contactor is inside the outboard motor cowling along with the controller and motor. Those are the basics at this point. In addition there are various gauges, motor/controller coolant system, DC-DC converter, etc.
Here again any ideas you have about this stuff would be much appreciated. For instance: My two battery boxes are 4' apart. Each has a 72v battery. The boxes are connected by a conduit. My plan is to connect those two batteries in series. Can I use one Orion BMS for this set up? (Note: I edited my original post I switched "parallel" and "series")