You have to request a print-out of the rental agreement.

Your Emerald Club master agreement basically is your rental agreement. In theory, you could put it in your glovebox as your registration.

I would prefer not to do this and always ask for a printed agreement. I noticed that the San Diego airport location has a sign saying "Please request a copy of your rental agreement if you plan to visit a military base". I definitely would not want to get pulled over and have no rental agreement in the car.