I installed the Ultra Breeze cover from Fantastic Fan. It went on OK, the roof is slightly bowed but I was able to make it work without any modification.
However, I did purchase the additional screen for the vent cover and found that the main fantastic cover, the one that moves up and down, interfered with the screen because the vent cover was slightly warped due to the curved roof. So I gave up on the screen since there's already a screen on the main fan and a second screen is really not needed - not unless you're expecting critters to hide out in your vent cover!
2015 30' Classic, "Chez Nu"
2014 RAM 2500 w/Cummins Diesel
ProPride Hitch, 400 W Solar