The battery no longer works but I’ve kept it plugged in and used it for streaming music at work the last few years. Today it’s in an infinite loop of rebooting and I noticed the glass popped out!
Well, it's not sand exactly but I had a lot of spent casting silica on hand.
It's basically a dirt filled metal ammo box under a pile of bricks. Even if it was unable to relieve pressure through the seals (no idea, but waterproof rarely is also airtight especially for internal pressure) I'm not concerned with an explosion. Although to be clear, the gasses produced by lithium can absolutely cause a sealed container to rupture.
The bricks are quite insulatory by themselves, the dirt was just so it could absorb moisture a bit better just in case, and if it all does explode, the weight of the bricks will keep it from hurting the garage door 30ft away. I don't believe there is any danger of hitting the road with anything should the worst happen.