The only way to choose your internal namespace so that it doesn't conflict with external names is to use a namespace you own, because there are no reserved domains for "internal" use.
Specifically, that's going to mean that your internal names need to be subdomains of a domain name you control, such as your organization's domain. Something along the lines of "*.internal.companyname.com".
I only see a bunch of reserved TLD and second level domains, of which non seems reserved for such use.
Can you elaborate a bit?