Hey Chaser, I gotta tell you. When I was a kid in the early 60s (I'm really dating myself), I went with my family on a trip in a station wagon, and they had bought a tent that hooked onto the back of it. That puppy was HEAVY canvas. It was meant so you could have the gate down and throw kids in the back in sleeping bags and then the rest had enough room for adults to stand up in and a couple of cots. It would also work stand alone style when you added a flap. It rained so bad one time, yet the canvas tent stood up to the worst weather and was perfectly dry.
So recently I went to see a neighbor's who had one he just bought for his SUV that went on the back, and he was testing it out just out in the yard. The material was so skimpy. It was light, which is what you want when you backpack, but when you have an SUV, you're not dragging it far, just out the back, and I thought it would be the perfect situation to have a decent heavy canvas one like the one I camped in as a small child.
Maybe they make sturdier ones, but if you can get one heavier, like I said, you don't have to drag it far. And having the flap where you can leave it in stand alone mode would be helpful too. I don't even know if they make them like that, but the skimpy ones look like they wouldn't take any kind of serious wind or rain event.
If they don't make them, I like bullfrog's idea.