There are laser engraving machines that are selling on Ebay for $71 now. They look like a little 3d printer and you can hold one in one hand.

This type of machine would allow putting keys and squarecodes on anodized aluminum plates or painted steel plates. They would burn through the paint or anodized layer.

I suggest this as a way to avoid all the issues of paper getting wet, mildewing, rotting, catching on fire, etc.

Although it's possible that an engraved steel or aluminum plate would not survive a fire with the engraving legible. Still a gigantic improvement.