Just released a minimal fullscreen countdown package. The full source code is available on github and you can install it from there or here by clicking the “run on info-beamer” button.
Configuration is minimal as only the font size and colors can be modified. Controlling the countdown is possible on the device page where a new interface pops up once the setup is assign to a device:
You can set a time in minutes and start/stop the timer…
… until it runs to the end.