Defend the hours you actually think in

Deep work does not get eaten by people being inconsiderate. It gets eaten because it is the only thing on your calendar that is not written down, and a calendar treats anything unwritten as available.

The fix is unglamorous: make it an event, like everything else that survives.

Decide when you are actually good, not when you are free

Most people book their thinking time into the gaps left over, which is exactly backwards. The gaps are where you are least able to use it.

Spend two minutes deciding when you genuinely think well. For most people it is earlier than the slot they have been defending.

Book it as real events

Book me two hours of focus time every Tuesday and Thursday morning at 9am,
Denver time, for the next twelve weeks.

Title them "Focus — do not book" and mark them busy, not free.

If something already exists in that slot on a given week, skip that week
and tell me which ones you skipped. Do not move anything that is already
there.

"Do not move anything that is already there" is not politeness, it is safety. An instruction to protect your time, read literally, can be satisfied by clearing your calendar, and the first thing you would know about it is other people asking why their meeting vanished. Say the boundary out loud.

Marked busy, or it does nothing

This is the part that decides whether any of it works. An event marked free is invisible to everyone checking your availability, including the employee you just asked to find a slot across six calendars.

If you have been booking focus time and watching it get double-booked anyway, this is almost certainly why.

Review what actually happened, monthly

Look at the last four weeks of my calendar. For each focus block I had
booked, tell me whether it survived, got double-booked, or was moved.

Group what displaced them by who or what it was.

This is the report worth reading. Focus time rarely dies to one big thing; it dies to a particular recurring meeting, or one person, or your own habit of saying yes on Tuesdays. You cannot fix that until you can see it named.

What good looks like

Two things: the blocks appear as busy to anyone checking, and the monthly review gets less interesting over time.

If the review keeps naming the same meeting, the answer is not better calendar defence. It is that the meeting and your focus time want the same slot, and one of them has to move permanently.