Do you have the one that came with the car? I have come up with a temporary solution using the OE plate holder, a block of dense packing foam, and some 3M Adhesive Putty.
http://www.amazon.com/Scotch-Adhesive-Putty-Removable-860/dp/B000AN7EW4
I keep it in the car and when I’m in SF and must park on the street, I’ll install it when I get near where I’m going to look for parking. You don’t want someone to see you do it so they won’t be tempted steal it. That said, I usually park in a private lot to avoid the BS.
The last time I got nabbed for it, I devised and tested this solution and drove to a local police station for sign-off. I had to park down the block and the officer, too lazy to go outside, said “is the plate on?” I said yes and she said “ok, i trust you…” and signed me off. The DMV can only sign you off if they were to blame… like you didn’t get your plates in time or something like that.
I’m sure you feel safer now, right?