Ironman523

You could use the 40mm pipe clip 'trick' to construct a plate assembly that clips on to the down bars in front of the gearstick and mount the sat nav on that. See my post here where I mounted a panel for clocks..

http://www.talkmorgan.com/ubbthreads.php/topics/180138/6

No damage or drilling to the car at all.
If you can't get the clips in the UK, I can post some to you from France.

Omne trium perfectum