use VMs. qemu/kvm. the Tor-based Whonix OS takes the approach of one VM running a Tor proxy and another VM running your application software. the latter VM only has access to that proxy, and no other network interface. it’s effectively the same approach as i understand a slug to be, but with the hardware virtualized instead of physical (or course you don’t have to use Tor — you can define whatever interface you want: a VPN, a firewall, etc).