HI Mark,
I tried everything (custom ones etc) they all sit on the car everyday rain, hail, shine, dust and everything else.
As D4D said the Rhino (which i know have) is by far the best. No dust etc. However i am getting a little water in mine, which might be because of condensation more then anything..
Otherwise when i replace this one it will be another rhino.
I did have a guy make a special sleeve for the RV-4 its out of a rubber canvas but i have to put the seam at the bottom when on the roof, and the back is velcro extra long that i double over a couple of times.. He also did double stiching with a double rubber over the original sticking at the front to seal it up. It works great to keep the Oztent dry.
But the rhino is working the best out of all the bags.