I like the idea of the Freewrite, but I would never buy one. It creates a sense of being a distraction free writing tool, but you could still check your phone, which is probably with you 99% of the time anyway.
There is a simpler solution.
It doesn't require you to carry a clunky box or even a dedicated bag. It doesn't depend on any electrical power supply. It's completely disconnected from the Internet and it doesn't require you to press any buttons.
Ladies and gentlemen, I give you the pen and notebook.