The hot glue was referencing if you were to use the slot already in the car, as a lot of the double din units have a single din rear. My HU is 9inches but has a single din rear and is able to adjust to either a center, top or bottom slot. Obviously since the slot is already there at the top I just adjusted it to fit there. If you were to have a double din HU that has a double din rear you would just screw the necessary cage for that head unit into the side as there is plastic for the screws to catch. This all depends on what HU you have and what size it is though. The opening for the kit is 7in but I went bigger to have more of that Mercedes "floating" look, like what's in the newer models. Hope that helps to clear up any confusion.