It sounds like you want some type of plausible deniability[1] or a way to hide your secure data in pain sight but often security through obscurity is a terrible idea.

Beyond using FileVault and a firmware password I don't think there's much else you can do besides physically removing the disk. Unfortunately that is not a easy task on the latest MacBooks.

1: https://defuse.ca/truecrypt-plausible-deniability-useless-by...