There are three ways that I can think off to store your private keys safely (offline)
1) Having a hardware wallet (Trezor, Ledger, and so on). The problem: Having to trust their custom RNG
You can add a custom password aka seed extension. If its entropy source is something like dice rolls and it's 128-bits of entropy strong or stronger, even with a bad RNG on the Trezor, your funds are secure.