That's one of the reasons why I love Electrum wallets.  If it came to it, it really isn't that hard to hide a sheet or sheets of paper with seed phrases on them.  There are many ways that could be done--and right now I'm actually thinking about something along those lines, even though I really don't have much crypto at present.  If you're ever involved in a crime, even if you're not the perpetrator, the cops can and will seize anything they think is relevant--and you never know what that's going to be.  Better to have those seed phrases hidden away from prying eyes.

If your memory is good, keeping a seed phrase in your noggin isn't impossible.  There are even artificial memory tricks to help, and with those you can be as creative as you want to be.

For other coins I own, the wallet.dat files are the best I can do, and so be it.  I've made multiple backups on flash drives, so hopefully that'll do the trick.  I certainly realize the importance of keeping all of this stuff safe, and the only relatively sure method I can think of is to bury it.  Sheets of paper won't be caught by metal detectors, though I'm fairly sure thumb drives might.  It's risky, but then again everything is.