Add Google Dynamic DNS to Sophos UTM

So you have a Sophos UTM device and want to setup dynamic DNS using your Google Domain? Sophos UTM offers a couple of built-in services for DynDNS, but Google Domains is not one of them. There are quite a few guides out there for hacking the UTM so that you can get this functionality, but they all seem to require creating a custom configuration file and then either manually starting the service at each boot, scheduling it to run, or something similar.
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