You can do this on the second level (actually the first level after the tutorial).
Set the difficulty to easy. Take the carpenter and barricade all the windows, then wait while the zombies build up outside. While you are waiting, break all the furniture in the room with the jukebox and search all the stuff. When you are done, you should have a big empty room, plenty of parts, and a few coins. Make sure to keep your barricades topped off as needed. At this point, you should have zombies piled up outside several windows. Switch to the boy scout and barricade the carpenter in the room where you originally found the kid so he doesn't get in the way. Now run around to all your barricades and undo them so the zombies get in. Lure the zombies into the room with the jukebox using yourself as bait. They are very slow and rarely strike. You might take a few hits, but it should be pretty easy to dodge them while circling the room, which tends to herd them in the center. When you get a dozen or more, build a toilet trap over an entrance that the zombies are still using to enter the room. As soon as an infected zombie hits the crowd and the disease spreads, put a coin in the jukebox. The zombies will die pretty quickly from the infection, so don't wait too long.