So an air-gapped computer+printer is best, and then protect the paper + computer from access and fire/water etc. No special stuff beyond that?
I ask because I want to store some keys that don't use BIP39/HD seeds to generate them.
You could probably do with coming up with something seedlike for the private keys in case you have a misspelling. Alternatively, you could write down the seed in a number base with few overlaps (base 8 or 10 might be good ideas). And make a checksum at the end just in case you mess it up...
As said, you can also use a bip38 password for enhanced security.