At work we do promiscuous pairing, https://engineering.itpro.tv/2018/12/07/our-first-experiment..., which encourages getting into beginner's mind (which is much easier to achieve, maintain, and recover when lost) instead of flow (which can often be difficult to achieve, maintain, and recover when lost).
It doesn't answer your question, but hopefully it's a helpful alternative for you. It has been incredibly helpful for us.
My trick is very simple and very low-tech. Take a piece of paper and a pen and draw a diagram that sums up the big picture of what you are trying to achieve (architecture of the software you’re writing for instance). Take as long as you need to, and when things start to fall naturally into place and you are eager to start, pick one part of the drawing and start working. Avoid distraction at all cost (music does help, especially in an open space).
I developed this when I was in academia and it transposed very well (better?) in the software industry. If you do that regularly it also helps course-correct when needed.
Worth noting that I find the flow state far easier to enter at different times in the day. Between around 2:30 to 4:30, I find it very easy to immerse in a task, to the point where even if I _should_ switch to a higher priority task, I will most likely stay on the track I'm on.
Who knows if that is circadian rhythm, conditioning, a combination of both...
It doesn't answer your question, but hopefully it's a helpful alternative for you. It has been incredibly helpful for us.