As said, the "Witches Hat" just moulds itself to the tunnel, helped by the thin edges. The rubber mat then shapes itself over the "brim. Best not to use screws but a square sheet of Dynamat or similar can have a hole cut in the centre and placed over the hat to seal it down to the tunnel. This method useful if the rubber mat has been replaced by carpet.
The first time I had the gearbox out of my wagon to do the clutch I found that the boot was held down with duct tape. Fairly neatly done but it was coming adrift in several places. I replaced it with a length of sound proofing felt over the top of the tunnel, two birds with one stone.