It's a $20 "piece of junk" but a portable word processor could keep you focused on your writing and nothing else.
Design for Hackers author David Kadavy explains several of the benefits of using a tool like this. It doesn't connect to the internet (so no tempting distractions), but can plug into your computer via USB to download your writing. Because it only shows four lines of text at a time, you only focus on what you're writing, not what you just wrote.
And because editing sucks on the device, you don't edit while you write. You just write.
The AlphaSmart 3000 is discontinued but still available for purchase from various online stores. There might be other similar old school word processors you can use instead. Of course, there are lots of distraction-free writing applications you can install to get a similar environment, but having a distinct physical device just for writing might make you more productive.