Cheap ones tend to be a nightmare to quickly install and get off again, so much so that most people tend to put them on and leave them on or just give up on them after the 1st on/off cycle. We use a Thule one and it takes around 1 min to get on and off and folds up when not in use and takes up v little space, but it is one of the really pricy ones. Thule do do cheaper ones and I've recommend starting by looking at them.