We'll be packing up our gear again in our Roof Bag this summer, and I just purchased the tire step -- the one accessory we didn't get last year! Worked wonderfully for a rain-filled camping trip (travelled 600 miles round trip in rain and high winds).
-- to make it easier to access the bag for packing and unpacking. We had some noise early in the road trip, but made a strap adjustment to remedy that.
It is very strong and the zippers are well protected.
There was much more room if we would have needed it. There was absolutely no noise while driving on I-95 at 70 mph.
I also purchased the protective roof mat to make sure the roof wouldn't get scratched or marked. I made sure I tightened the straps so there were no loose straps to flop around.
Didn't know these existed until searching the internet.
We chose the waterproof versions for our 4 door sedan though there was nothing stating you should or could mount one to the back trunk/window of the car. I have used my Roof Bag or just the straps for strapping things to the top of the car (without a rack) on average of at least twice a month for nearly 5 1/2 years now!