You are wise to be concerned but I think you will not design an “enclosure” that will prevent further damage if it is inside a wood frame house. There is a house near me that burned down because of a single power brick for a tool in the garage.
Your better approach would be to make it so that power delivery is monitored and regulated automatically. That or manually just unplug after charging.
Thermal runaway happens on over-discharge or overcharge of the battery pack. Each pack has smart circuits to regulate this but they are designed at scale with a squinty eye at overall product cost…cheapest circuit available gets used even by well-known manufacturers.
Place power draw monitoring on the delivery circuit and when the draw lowers to maintenance level (batteries have recharged) have the power delivery automatically cut off. This can all be automated with freely available smart home products. You can even get some temperature sensors to monitor for overheat conditions during charging.
Yep, automation is part of the plan, as is additional smoke detectors.
I did look at basement ventilation too and I’m not sure I can do much there since the house is stucco over block (only the interior walls are wood frame). I have an existing vent fan out a window well, but I’d need to figure out how to weather seal it if I wanted to automate that …. Plus there’s no way it’s sufficient for a battery fire