If starting with http:// or https:// is actually required, I recommend updating the documentation, which does not say that. Currently the page you linked to says:
- “Namespaces are arbitrary identifiers, so technically you can call your namespace anything you want.”
- “using the URI of a resource you control is conventional” (emphasis added)
The page recommends using a URI that you control in order to avoid clashes, but nowhere on that page does it say that using a URI is required. Confusingly, when I’ve tried out custom rules with non-URI namespaces, their data is included when testing in the dashboard, but it’s not included in the data sent to the client.