Comment by desman

yep, I have brick walls downstairs where space is more scarce, the attic has lots of space but also lots of wood lying around.

I guess another possibility is to have the battery pack outside, attached to the brick wall, and ideally away of direct sunlight... but then no wooden roofs. It looks like no DYI is really feasible except maybe a concrete bunker as others have suggested. And even then it's probably uninsurable which is considered a no-no where I live.