In case you haven't yet set up an encrypted USB drive yet, this article has clear directions on how to encrypt an entire USB drive. If you don't have a USB drive that you want encrypted, please talk to your anchor or send an ask ticket for a USB drive that you can dedicate to this purpose.
From there, you can follow GitHub's directions for adding a new SSH key to your account, making sure to save the key on your now-encrypted drive (not the default location in your home directory). Please use a password different from the USB drive's password and different from your GitHub account password. (Since you're already in your account settings, this would also be a great time to enable two-factor authentication on GitHub as well.)
Then, to make it super-easy to just plug in your USB key and activate your SSH ke