Learning From The Real Thing
Your first task is to go back to the libraries and take a look at real historical evidence. This evidence doesn't have to be related to what you're trying to lie about -- the idea is to start generating some ideas about how to create a GREAT fake piece of evidence by modeling it after REAL evidence. The more you study the real thing, the more convincing you can make your fake thing.
Some good places to start looking at evidence might be in Special Collections, in Microforms, or in the art library.
Also, just a gentle reminder that those friendly librarians are still eager to answer your questions about research and would be HAPPY to help you out...