From what I understand, Water does not extinguish lipo fires. Sand is a better bet I think

I charge and store my lipos in my fireplace (which I dont use of course )

your other option, much simple than the system you are designing is to get a big, thick and deep clay pot. You might want to open a hole for the cables so you can charge inside of it

I think the name of the game is containing the fire, not extinguishing it