I realize not many people will bother, but it doesn't seem
that impractical. A small safe deposit box costs about $20 per year, and a U2F-only Yubikey costs $18:
https://www.yubico.com/products/yubikey-hardware/fido-u2f-se...Many sites don't support U2F yet, but that's improving. An alternative for Google Authenticator sites is a set of one-time codes, which can also be stored in the safe deposit box.
I don't exactly see the point of using another site as backup, since you'll want 2FA on that site as well.
