Generally the trick with that "design an X" question is not to design anything but to ask the interviewers questions. The trick, such as it is, is to gather requirements as opposed to actually knowing how to design anything. I personally feel it's more of an object lesson rather than an interview question - it's more Socratic than anything else. Other than that the process is a crap-shoot. If you've read Mt Fuji there's not much left to say. Interviewers are capricious. My only suggestion is that you have the read the interviewer to try to get a sense of what they want. This isn't always possible though as borderline-Aspergers software developers don't exude body language. Kinda the opposite.