Here is a quick model I made that can be made out of sheetmetal (aluminum would be best to keep it lightweight).
I need to get some foam to get more accurate dimensions, but this is the general idea. I would paint it (probably black) and add rubber to sides to prevent scratching of the car interior, as well as rubber to the two flanges that hold the top of the nexus 7 in place.
Let me know your thoughts and I can go a different direction if you want.
Cheers