How I created this is as follows:
I took the case apart and soldered momentary-on push button switches into the solder pads on the circuit board. The switches and the screen were mounted to the backside of an OEM cupholder blank. I used a dremel to cut out a rough square shape for the screen, and used mini files to file it into a perfect rectangle. I marked the locations for the buttons and drilled holes that were slightly undersize, and then used a tapered reamer from the backside to get a perfect fit for the buttons.
It took alot of prototypes and trial and error, but it wasnt too hard. Dont worry too much about messing up the SG, because the company that made it, linear logic, loves it when people re-engineer their SG to fit in their cars. They provide free replacement parts and free repair, even if you broke it while messing with it. In fact, when I sent the prototype back about the 4th time for repair with a little note explaining what i was trying to do, and a technician there re-soldered my fine gauge wire to a stronger location on the board; it really helped me out.
Another great spot for the scangauge would be right below the center air conditioning vents. Just relocate the flasher somewhere else, and dremel/file the shape for the SG and mount it there.