Creating a USB drive that will boot natively from a Macbook?

Hello.

I am trying to install Ubuntu to a USB drive which I will then be able to plug into my (or any) Macbook/Mac and boot from using the native mac bootloader. Is there any methodology for doing this? I've found plenty of instructions for supposedly making a ubuntu USB boot disk in OSX but when I follow these my created disk doesn't show up at launch when I hold ALT like I usually would to select a boot disk.

Any help greatly appreciated.