This metal wallet looks smart. Engraving private keys is not going to work. The best way is to supply multiple sheets of letters and number. The owner insert the letters into the metal wallet himself.
The price is going to be an issue. It is not cheap to produce metal wallets and shipping is not going to be low.
I thought about this myself but how would they lock in there permanently? A good shake and your private keys are gone? If it's in a box and takes a tumble your coins are gone? No way, way too stressful, I think the engraving is better than that.