I did something a little different than usual, and skipped the the plastic bricks altogether. I bought this stuff called
for $6. Cut a small strip of it, and placed it right above the CD deck. The phone literally doesn't move. AT ALL. this stuff had excellent ratings and it makes a great place to sit the phone to use the GPS. I lay mine landscape.
I attached the Picture of what mine looks like. BTW if the iphone looks a little strange it's because I dropped it in the image, but I wanted to give you an idea of how I used it.