Hoss , Mountian Boss Hoss sells a rain cover made to fit the Boss Hoss. The main problem with hauling a bike with a cover over it is if the cover isn't secured so that no portion of it flaps in the wind , it will beat your bike to death . Meaning if the cover flaps against your paint or chrome the friction from the cover flapping against it can wear whatever it touches. Probably the only thing you can do to prevent this is to first cover the bike with soft blankets then the cover and still secure the cover as tight as you can all over with rope , twine tape etc.
I saw some covers @Mountain's website before. They have two types - one is a dust cover and the other is a rain cover. I've never seen one in person or used one ... maybe somebody else has? I am guessing if Mountain is selling them they have to be at the very least "decent".
I have the one from Mountain. The one for rain etc. Looks good, works good. You'd better wrap everything REAL good before putting that cover on before you trailer. It really will beat the **** out of your paint etc, if you don't!! Good luck!