Convert an old phone into a display for what you're doing.
Essentially a webpage to display an actively updated status fetched from a server. Host the page on a server and launch it on your phone, mount it to your door and update the status from your workstation so people know if you're available.
Preferably use with an Apache 2 server.
As of April 2020, I suggest using Firefox instead of Chrome on the display side, since it doesn't drain the battery or crash spontaneously when left on for an extended period of time.
Make set.sh executable (755), and status.json writable (666).
noSleep.js is cloned from https://github.com/richtr/NoSleep.js/ and available for redistribution for a different license under the MIT License.
Favicon made by Freepik from www.flaticon.com.