revkev has figured it out.
A self-supporting regular tent that will fit inside the area of a full length roof rack with a plywood floor is an ideal solution if you're not carrying stuff on the rack and want to sleep up there.
On short trips, where I didn't need to carry gear on the roof, I often slept in the past in a regular self-standing tent that fit inside the roof rack.... With the four corners tied down of course.
- Plenty of room
- Sets up very quickly (if you've got a good tent)
- Only costs a few hundred bucks
- Weighs almost nothing
- Stores out of the way
After I'm done sleeping in the tent for the night, I would de-tension the two poles that hold the tent up and it would collapse flat on the roof with my sleeping bag inside. Then I'd throw a tarp over it to protect it all from the UV sun during the day. Also detensioning the tent flat keeps it out of the wind if the breeze comes up.
I've slept like that on my roof dozens of nights. Very cozy.
The main benefit of sleeping on the roof is to get a flat floor and get off the hot ground... or cold ground.
It also gets you away from ants that seem to be able to gnaw through tents, and away from scorpions that like to bury under it while you're sleeping at night.
In bear (or big critter) country, sleeping on the roof is good.
What I don't like about any roof top tent, fancy or primitive, is that it creates "visual beacon" that makes the vehicle a lot easier to spot from much farther away.
Where you might be camped next to some boulders or brush that kinda hide the car or at least make it a lot less noticeable, a big ole tent on the roof during the day can make your vehicle visible from miles away.
In a designated camp ground or safe national park that's fine.
In other places, it's just asking for trouble.