I did some searching on the site to see if something had come up before, and people do recommend the Dowco cover - but I'm not seeing any size recommendations.
Dowco does have a chart and they list a CB550 Nighthawk - but the CB550 isn't a 'Nighthawk' if I'm not mistaken.
But ultimately it gets back to the main question - which size does fit the CB550 - the Large? For the sake of argument, let's just assume a Dowco Guardian WeatherAll Plus.
