I'm using the Bracketron Universal PHV-202-BL GPS Grip-iT Rotating Vent Mount - Black.
The Nexus 7 will not fit in landscape, only portrait (barely with the foam padding)
What's great about this bracket is that it can be used for other devices and is the first vent mount that I've been able to clipped onto and off the vents easily.
The side and bottom arms are adjustable also.
The picture on the left is from the web & the one on the right is my unit.
(I don't use the square base in web pic)
I think I got this at Radio Shack but a web search should find other online & brick and mortar locations.