Since I am looking at this thread again - I thought I would elaborate on this comment. If you search for "Lobo shelf" you should get some good results from thenewx.org and several other sites.
I have just started looking there, but this thread seems to have a good run-down of what they are doing. A lot of the links are broken but most of the pictures seem to be working.
http://www.thenewx.org/forum/showthread.php?t=1120
Xterras have some roof hooks that make it easy to attach with, but I am sure someone here could come up with something that works too. Maybe the 3rd row seat belt bolts? Add an eyebolt or something there for the rear attachment points? What's the thread size on those?