Insellium (btw. now full member of the Q3Rally Team) found a nice 2-Map-Pack for Q3Aaround the Internet. He tweaked the .arena file an fixed some shaders to make them work with Q3Rally. It is a lot of fun to play on these maps with up to 4 Players. Maybe 6. 😉 Author of the Map-Pack is Emelis (firstname.lastname@example.org).
You can find it, as always, in the downloads section.