If you don't want to cover it in plastic, just grab a large box and throw it in there...or find one of those rubbermaid containers...there should be one about the right size....it'll keep the rats out, keep most of the dirt out, and allow it to breath somewhat... Of course, there's always the option of strapping it to the roof of the vehicle, it'll keep it out of the way, and you can use it when you really need to. (Just make sure you include some goggles or something for whomever is riding up there...
