[Chad C. Mulligan]

One form of the “fire safe” I saw advertised was a bag made of fiberglass cloth. I don’t know if they are still marketed, but you could easily stitch one up out of a fire blanket as sold on Amazon. You’d want a separate bag for each battery.

https://www.amazon.com/s?k=fire+blankets&i=tools&crid=Q6BMZ0KWRUSA&sprefix=fire+blankets%2Ctools%2C2057&ref=nb_sb_noss_1

https://www.amazon.com/s?k=welding+blankets&i=tools&ref=nb_sb_noss

Ammo cans are still a possibility if the batteries you are thinking about will fit into them.

[4Liberty]

Thanks for your advice!