Syncing mailbox.org calendar to Google Calendar as anonymized "Busy" blocks

I wanted to share my mailbox.org calendar with a Google Calendar, but only show "Busy" time blocks — no event titles, descriptions, or locations. This is essentially what two Google Calendars can already do natively when you share a calendar and set visibility to "See only free/busy."

I managed to replicate that behavior with mailbox.org as the source, using a free open-source tool: GAS-ICS-Sync (Google Apps Script): https://github.com/derekantrican/GAS-ICS-Sync

Why this is worth the (small) extra setup effort, compared to just subscribing to the mailbox.org ICS link directly in Google Calendar:

• You control the sync frequency. A native ICS subscription refreshes on Google's own schedule (which you don't control and which can lag by hours). With this script you set it yourself — e.g. every 15 minutes or hour.
• You control how much detail is shared. The script is plain, readable Apps Script code (Helpers.gs), so it's easy to tweak what gets copied over. For instance, you can strip all-day events entirely, or replace every event's title with just "Busy" — giving you the same free/busy-only behavior Google offers between its own calendars, but from an external mailbox.org source.

Happy to share more detail on the code tweaks if anyone's interested.