You may find it advantageous to first create a subdomain under /NET/, e.g. /NET/mysubdomain/, or create a top level domain, e.g. /mydomain/, and then use that subdomain or top level domain as a privately controlled namespace for the rest of the PURLs you create. Another option to consider is to create a partial redirect PURL to point to the root of a remote hierarchy of URLs.
Of course, you can create PURLs directly under the /NET/ namespace if you so desire. All of these options are described in the PURL FAQ, so we strongly recommend looking it over before creating your first PURL.