I've been storing TOTP codes in on my Yubikeys for many years now, it works pretty decently. They have an app for it (https://www.yubico.com/products/yubico-authenticator) which has gotten better over the years. For redundancy I've stored keys on two devices.
As it happens this has been in the news quite a bit, and we've been told that the Finnish trunk lines are built for this [1]. Finland isn't split up into multiple electricity price areas unlike its neighbors.