I just got two for my truck. After using outdoor covers for over 15 years now, I've realized one cover does a good job, but two works that much better.
A 4 layer Block-It (Evolution 4) underneath, and a 3 layer Weathershield on top of it. The same combo has been keeping my Syclone clean and dry for years now. Plus, the two covers provide mucho protection should something bump into the truck.
I also have a Dust Cover on my Cobra R, which sits in a garage out back. Obviously not as much protection against bicycles and such, as it's much thinner material, but I AIN'T got no kids 'round here!