[raztogt21]

At my old company, under extreme cases, I put up a physical whiteboard on top of my desk with "Focus mode: on" and "Focus mode: off" depending on my mood.

Worked great, and nobody seemed offended. I may think that even the directors love it when they saw it.

[joejerryronnie]

I had a coworker who put on a hood with giant bunny ears when she had heads down work to concentrate on. You knew not to disturb her when the bunny ears were up - and it was very effective because you could see those huge ears from across the office so nobody wasted time walking all the way to her desk only to find her in focus mode.

[davman]

Total lifesaver: https://luxafor.com/ (disclaimer: I backed them on kickstarter)