I have a DC2 at the moment, and have an Alpine HU. The HU basically slid into the casing that came with it and that went into the factory hole. We also used the supplied bracket so that the HU could be mounted securely to the rear so that it didn't slide further in (if that makes sense).

We then secured it down the side with 2 screws per side from memory and then bolted it from the rear as well. Mine has a few extra screws in place for security but basically if you have two per side and one from the rear the HU won't go anywhere.

Give us some more info. What HU do you have?