Nice shiny seed plate that you engrave yourself on stainless plate that you can purchase inexpensively from trophy shop or tag making company. Add a passphrase to the wallet that has reference and meaning to them. Put this passphrase on a second plate so it can be stored separately from seed plate. (make backups)
Like their marriage, the two parts make it whole, strong like steel, impervious to the elements.
I buy stainless plates custom cutwill be like 3X5" for less than $10 ea. They can then get a hardware wallet any time in the future.