[kirushik]

Another possible take on this would be Parity Signer (https://www.parity.io/signer/), which is an opensource app turning your (full-disk encrypted and put into an airplane mode) old phone into an improvised hardware wallet.

Very narrow attack surface (no USB or Bluetooth, only QR codes are used for communication), and more safety against the supply chain attacks make it a viable alternative for some particular threat models.