.home.arpa is meant for this. It's supposed never resolve to any internet address so even if a DNS query ends up outside your network (like if an application handles DNS itself) it won't resolve to an internet address and send data there.

You can add .home.arpa to you DNS search path (configurable with DHCP) so you don't have to type it, e.g. if you look up "nas" your computer will try "nas.home.arpa" too.