I personally find the added wind resistance of carrying on top and the resultant increase in fuel consumption put me off a ‘permanant’ roof mounted carrier. The fuel and speed goes down noticably with the bikes on the roof.
When I can, I carry them in the boot and use an old duvet cover to wrap around and wash it often but, when I want to carry them on the roof I have my own little system which doesnt use actual carriers.
I have foam covers on the roof bars (pipe lagging taped on would do), I then have 4 ratchet straps which I cut real short. The bikes go upside down with the bars along the roof bars, one forwards, one backwards. I then ratchet the grips to the roof bar each side, and finally put a third strap around the other roofbar and the upturned saddle.
Its rock solid, pretty quick to do(although not as quick as a dedicated carrier obviously), cheaper, and produces less wind-resistance than a couple of carriers in addition to your roof bars….
I just whip the roof bars off and stick them in the boot for non-bike trips. Saves petrol.