Get a flat scrap metal
Metal scrap is not really a cryptocurrency wallet unless you are some magician who can do this alchemy...
You can use metallic plates for backing up seed phrase words but only if right material is used like stainless steel and copper, because all other metals are worthless and can be destroyed in fire.
Gold will hold in fire and it would be interesting to use it as metallic plate and make it even more valuable, but you would probably be crazy to use it for this purpose.
Jameson Lopp tested many metallic plates and not all of them pass the test and similar thing would probably happen with your random metal junk.
Once the coins are spent, dig up the metal and cancel the spent keys.
You can't cancel the keys, and it's not easy to destroy metallic plate made from stainless steel or copper, not to mention that you need proper tools for doing this.
One alternative and cheaper way would be to put regular paper wallet inside certified fireproof bag.