Need Hidden Camera Help To Watch Wife Masturbate At Home While I'm At Work

My wife masturbates while in the bathtub and I want to be able to watch her do this while at work. She thinks it's incredibly hot and is OK if I put in a hidden camera. We don't want to use a foscam or something like that. She likes the idea of not really knowing that it's there since she won't be able to see it nor will she knows when I put it in.

Does anyone have any recommendations on setting one up?