Shopping List: A form of sheet metal, (galvanised steel etc) an engraver a pencil and ruler and an hour or so.
Transfer your coins to a paper wallet that you have rolled offline, engrave the private key onto the sheet metal (store in a safe place until needed). Destroy the paper wallet once you have verified what you engraved is what you intended to have engraved.
Even better, easier and cheaper, is to buy a piece of normal cheap PVC pipe used in waste water systems.
Carve your private key or seed words to that, and bury it somewhere where no-one will be building new buildings or anything in the future. Preferable to your own land or your relatives land

PVC pipe is easy to carve and lasts forever in the ground. Metal detectors wont find it and house fires are not a problem, because the layer of ground will protect it from heat.
One way to quickly and easily dig it to the ground is to have a battery operated drill and one of those cheap 1m drill bits. Drill a hole to ground, and then push the piece of pipe (if you selected one of those 3cm diameter pvc pipes or a small piece of a bigger pipe.) to the hole, and then just fill the small hole.