Something similar to this: https://www.sentrysafe.com/product/CHW20201 https://www.sentrysafe.com/product/CHW20201 Main point is that it's fire-proof and water

Something similar to this: https://www.sentrysafe.com/product/CHW20201 https://www.sentrysafe.com/product/CHW20201 Main point is that it's fire-proof and water-proof, and you can bolt it down. I'm not expecting it to protect against high-motivation thiefs, but at least (hopefully) survive the house burning down or being flooded, and you can't just grab it and go.