Hardware Router For macOS
If you want more security than what you have now, you could add a hardware based router/firewal between your roommates Internet connected network and  your "network"

What you would be doing is connecting your WAN port of your router to your roommates LAN.  This will create a "double NAT" network, where your roommate becomes your ISP. 

The benefit this has over a highly configured firewall is that the router will immediately block all incoming traffic (by default) on the roommates LAN but allow you egress to the Internet.