One thing, though. I would use a good quality spring between the whip and the ball mount. I didn't want any shocks (impacts) conducted down to the mount.
The other thing is that I used tie wraps to hold the longer antennas in an NVIS position. The tie wraps (like a chain for insulation) would go between the whip and the luggage rack. I've also used 50# fishing line as an insulating tie-down.
I'm not doing any major off-roading or anything like that so I'm not worried about shocks. I don't like the look of the spring anyway.
I'm using a nylon tie to lock down the whip with some dacron rope as well. The tie goes around the ball (which is tight) around a loop in the rope. I can adjust the rope for height. It's about 4" off the rack -- which keeps the whip under 19" -- which is under my VHF antenna that is at 45 deg angle.