I work in IT and have an Open Plan office.
Recently with our HP Laptop purchases, we've been getting these Security Screens which go over the screen of the laptop, and you can't even notice it's there - They are about 1mm thick.
When you move your head slightly to the left or to the right, the screen appears as if it's completly turned off.
It would be extremely easy to trim one of these bad boys up and just stick it into the plastic around your number plate, would work an absolute treat i reckon.
Like these:
http://www.secure-it.com/shop/index.php/cPath/38_41