How do you stay focused during deep work?

You've inadvertently given yourself the best advice for your situation: get rid of the notifications (let alone constant ones) as you do deep work; more generaly, establish and defend boundaries in space and time. Also, if you haven't, do read Cal Newport's "Deep Work."

I struggle to focus at work sometimes, especially after the most of the day is done and I have this awkward hour or 45 mins left before heading home. However, somehow I always end up barely making it for the train back because under a pressure of the deadline (or more like the train timetable) I get into a hyperfocused state of high productivity. So 1: set an end time.

Another thing for me is that I tend to procrastinate tasks that I don't know how to tackle, not necessarily from the technical perspective but also from the conceptual side. Hence my number 2: face that you might not know how to start. Either I need to do a conscious effort to crisp out what the issue is or have a discussion with a coworker.

Also, my number 3: no notifications on my laptop. Yes, I have missed meetings because of that but that's a risk I'm willing to take.