Or you can just get a Metra kit that will give you a better (more secure) fit. I did install the original pocket when I had installedd my old Clarion back in 96. I did a better job than shown on that site though. I actually managed to use the metal clips on the pocket. Though when I installed for a friend later that summer, I did the duct tape trick.
This time around I have both a new Clarion head unit and the empeg, so there's no pocket. I did use duct tape with the cages of the two units though.
Bruno