I saw this one from a bicycle mount. I wonder if you can make it work on the motorcycle.
Let me know if you find something better. I too ride and it would be nice to have something. What I do now is put it in my pocket and turn on tomtom. I let the voice tell me where I should be going. (perhaps its safer to do this then mounting.)
http://www.valuemax.co.uk/product_details.php?id=16173