I was just googling for mention of projects involving Arduino and Lego when I stumbled upon this picture of someone else having done just what I did. Replete with the same rubber mat. And sharpie. And green rubber-band.
How lovely to see my tomfiddlery with your motor shield come full circle.
This is awesome! I regret giving away my old Mindstorm kit. T_T. I wish the creator would include build instructions though.