suppose you have the following tools available;
1. Two trezor hardware wallets
2.offline bootable pc with ubuntu
3.bitaddress.org siteburned to a CD
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.
The problem with that is that the cost for doing it is too high, and it might not be worth it if it made up any significant portion of your worth im bitcoins. On the other hand, we could simply keep separate and several backups in safe places and the store a brain secret to decode the seed or private key. This way, it is very unlikely that the coins could be obtained by anyone else or lost by the owner.