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=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.
Thanks for your advice!