Good idea for a mount location. I use RAM mounts for all my vehicle mounted equipment, so already have a bunch of mix and match components and device cradles. They are incredibly solid and adjustable, and once you've got a mounting solution for a new vehicle, you can mount any other device you have an attachment for.
For this, I'll just put an inexpensive RAM ball (a few dollars) on the end of a threaded rod adaptor - should work perfectly!
This is RAM's GOPRO mounting system.
http://www.ram-mount.com/NewProducts...5/Default.aspx