Roof top tent and related?

What kind of roof top tent works best on an FJ?  I've seen a lot of hard shells - lower profile - may save a little on gas mileage?

Do you need to replace the factory rack with a rhino rack or similar?  If so, what's the best place to go for same?

Finally, the horn on an FJ is modest to say the best.  Anyone have thoughts on something with a little more presence?

Thanks, Brian in San Rafael