Really wish there was a basic set of commands for manipulating namespaces. It seems we are reliant on support being baked into larger tools like systemd at present, and said tools may not allow the user/admin to manipulate namespaces directly.
Linux namespaces were created to support containers. This is how userland container projects like LXC, Docker and Nspawn work, only they don't use the unshare tool but the underlying system calls clonens, setns and unshare [3].
[1] http://man7.org/linux/man-pages/man2/unshare.2.html
[2] https://www.flockport.com/alternatives-to-docker-and-lxc/