Y’know - I think stacking cinder blocks and some pavers on top would actually work just fine. Just angle the cinder blocks so the ones at the bottom have the holes poking out (and into the enclosure) and that should provide enough airflow to keep it from getting too hot in there.
Heck, you could probably angle them all that way except for the top ones. I haven’t seen videos of battery fires, but unless they’re like really really flammable, the fire shouldn’t burn out from around the several inches of concrete that surrounds it.
Remember fire burns up so the pavers and concrete blocks at the top will be what are actually blocking the most.