Since other threads have pointed towards M3 being the size that the torx screw it, I went to looking through my computer parts bin.
The screw used to secure CD-ROM and Floppy drives is the exact size needed. An M3x5mm
Pull the rugged mount apart, put in the new longer screw. I didn't bother putting the clip back on the screw. Its about 1mm longer then the garmin screw and will secure the Montana easily using a philips screw driver. I didn't like to cleanness of the other methods.