best file encryption software

What kind of compatibility do you want?

Creating an encrypted disk image is quite simple: How to create a password-protected (encrypted) disk image

If you want to back it up efficiently with Time Machine, you need to use a Sparse Bundle disk image as the format. Otherwise, the entire disk image gets backed up with any small change inside of it.

If you need to access the files on another operating system, I don’t know what third-party solutions exist.