When I was still teaching law, I used a free app called
DarkRoom to write my lectures and essay exams. You can use it on Chrome, Windows, or the iSheep version WriteRoom for apple devices.
It displays a classic black screen with a green text like an old interface. you can change the size, color, and typeset of the font as well as the background color. You can set the page width (even when working in fullscreen) so as not to have 3000 characters in a line if you have a huge screen.
The app completely full screens whatever device you are on, so you cannot see anything else. As long as your volume is off, you won't get any alerts, etc. While I really like the notion of a dedicated device like the Freewrite or Alphasmart, I think that a smidgen of discipline and the right freeware can prevent the need for additional purchases.
My wife participates in NaNo every year and writes some daily. I offered to get her a Freewrite or comparable device, and she opted for an old laptop with the internet turned off.