You use the original speaker mouinting posts but because you're stepping up in size you need 2 MDF rings. One 9mm ring cut to fit the 4" OEM opening, and fit over the OEM mounting posts (as Audi kindly don't have flush fit fixings), then a 6mm ring cut to fit the 5.25" new speaker size, with slight notches cut into it so you can get to the screws to the mounting posts. The 2 rings are then screwed & glued together to provide a semi solid mounting plate for the speaker. The job is slightly complicated by the fact that the 3 A4 mounting posts are at right angles to the door card, but the aperture is angled upwards, so to get a flush fit to the speaker hole the 9mm ring has to have slightly angled holes and the position of them is quite tricky to get right.
Well worth doing if you're handy with a jigsaw as the whole assembly does provide a more rigid construction than the plastic off the shelf ones that need a whole bunch of silicon to seal them properly and prevent the speaker flapping in free air.
I was lazy and didn't rewire the doors, tried to with a wire coathanger but gave up. If you do rewire the doors then you can mount the crosovers behind the glovebox.
