A few years ago I was stopped in my Speedster for not having a front license plate. I didn't have it in the car so the officer gave me a $50 ticket. He said if it was somewhere inside the car he would not have given me a ticket. It's been under the passenger-side floor mat ever since without any stops.
If you're willing to take a chance on getting a $50 ticket, put the plate under your floor mat. If you get stopped explain that you just haven't had a chance to install it yet but have it with you.