The Cavity:
For what it's worth, we're doing the same thing with our current project. To help avoid the debacle that Jungleguy/Machead referred to, you'll need some scotch tape, scissors, a ruler, and a thin piece of plastic. Oh, and a calculator if you're not good at math.
Cut the piece of plastic so it will completely cover the LCD panel on your camera. Use the marker to place dots on the piece of plastic where the corners of the LCD panel are. Next, do the math and figure out where the top and bottom boundries are going to be for your 16:9 letterbox. Using the ruler, draw the top and bottom boundry lines on the piece of plastic. Line up the dots on the LCD panel and tape the piece of plastic down. Frame accordingly.