Alternatively, you can leave them sit out in the hot sun (or some of us lazy people have been know to use the wife's closthes dryer for a few minutes) to warm them up a bit. Make installing them a whole lot easier. You will need to re-tighten them a few times.
Needless to say, make certain you've got a real good coating of wax or paint protectant on the lids first. I always take my vehicle bras off and clean underneath the covers every few hundred miles, especially if you ride in the rain. Nothing worse than finding some road grit has gotten under there and acted like sandpaper on your paint while you thought they were protected.