So, I've build myself an app to make that easier. Essentially, it's just a more stupid version of a text box. It's free, it's private, and it's meant to put you in the state of flow.
I've been using it every day for the past 3 years or so and I know that some people find it useful too, but even if I was the only user, I'd still be quite happy with it, since I suck at sticking with habits :)
Check it out!