Is there a general rule-of-thumb on how to name package namespaces for open source projects with no own domain?

Namespaces should be unique, so domains were chosen to take care of that, but in the end it doesn't matter, as long as they are unique.

Now, if I have a project that is only hosted on github, is it okay to take

com.github.username.projectname

or is that rather frowned upon, because I use a domain that I don't really own?

java namespaces
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