I removed the mounting brackets from the factory radio and mounted those to the aftermarket mounting sleeve. (I used pop rivets and then hammered them down to make them as flush as possible.) The Dual unit slid into the sleeve and locked into place. I then used the factory mounting screw to attach the new radio to the dash.
The Dual unit came with a trim piece that nicely filled in the gap around the sleeve. I also used 1/4 inch washers between the bracket and the dash to "fine tune" the mounting depth. (Stack as many washers as necessary to get the correct depth - I used two on each screw.)
I'm really particular when it comes how well things fit. My unit fits snug and looks great. Also, I have experienced no squeaks, rattles or creaks in over 3000 miles of use.