I second the recommendation for the ARB 2000 model. It's 2000mm long (6'7") x 2500mm deep (8'2"). The 6'7" length fits the Xterra perfectly and doesn't really protrude on either end, even with an OEM roof rack. That is the same model that both Toro and I had on the Rattlesnake Canyon run, just to put it into perspective for you Barb. It can be had for about $275 shipped via eBay or Amazon, and the quality is top notch (that's not a knock on any of the other brands, because I haven't owned any of them to compare. But I can say this ARB unit is solid and built with high quality materials).
The easiest way to mount it to the OEM rack is to get your hands on some Yakima X Tower mounts. They would allow you to clamp it directly to the OEM roof rails while moving it up and away from the rail about an inch, ensuring you have plenty of room to swing open the lid on your wet box still. Unfortunately, getting your hands on these mounts isn't easy, as Yakima stopped producing them a few years ago. They pop up on Craigslist and eBay every now and then, and you might also have luck posting a wanted to buy ad on TheNewX. You'd need three of them to securely mount it, but might be able to get by with 2 (I would strongly recommend 3).
Here's a write-up from someone who mounted this same awning using the Yakima X Tower method:
http://www.thenewx.org/forum/showthread.php?t=100585In lieu of the X Tower brackets, you could make something using a few exhaust clamps and some L brackets or angle stock.
