No, I didn't added the Etna suggestion, yet.
Oh nice, there is still hope for Etna special edition 
Honestly, the raw material cost is nothing. 4€ for washers and a few euros for the stamp
Are you sure it's still 4€ in all places?I heard $5 wrench attacked is now renamed to $10 wrench attack because of inflation and price increase
Maybe it's good idea to stock up on steel washers as I am expecting price of metal to go up more in future, and you can always use them in real life.
I didn’t explicitly mention derivation path as I didn’t want to mess up things. Maybe it’s better to state the obvious? “Don’t use any fancy wallet to generate the seed? I should include a derivation washers”? Might be unintelligible by the person who should get the Satoshi?
I talked many times before and importance of derivation path especially if you are using hardware wallets with BIP39, some of them are totally different by default.If you only use Bitcoin there is less chance of having issues in future, but add any other shitcoin and you could be having a lot of issues.
Best thing would be to add name of the wallet (also important if you used Electrum for seed generation) on one of the top washer.