Ask HN: How to synchronise free/busy between multiple calendars

Hi, I've been looking for a solution for a problem I'm facing and I'm sure I am not alone.

I have 2 Google calendars, 1 iCloud, and 2 Office 365 calendars. What I'd like is for an event in one to be copied to others but with no details (just 'blocked').

Is there a service or Mac app that can do this?