All I wanted was just:

• an USB key with Ubuntu 16.04 LTS installed on it,

• with /home and swap encrypted,

• that I could boot from any of my 2 Macs, by simply pressing and holding the “option” (or “alt”) key immediately after boot,

• without messing around with their respective boot loaders.

After trying dozens of solutions on all available blogs on the Internet I finally found THE solution that worked for me!