Veteran locksmith Tom Rubenoff
says you can bring an old mortise lock in and have a key made for about $30—or make one yourself from a skeleton-key blank (about $6; antiquekeys.net
). Remove the lock from the door, and unscrew the lock case to expose its interior workings. Note the wards—the bumps on the case beside the keyhole—and the locking lever and bolt directly above it. For the lock to work, the key's bit has to turn past the wards, lift the lever, and engage the bolt. Put the key in a vise and use a warding file to cut notches into the key. Start small and test often until the key works the lock.