Agree on the heavy tarp. Silver or white extra heavy duty. Also, make sure you add chafe protection. Any projection, even rounded will punch a a hole in any poly tarp. I stored this way for three years and replaced some tarps anually. We now have one of those super nice canvas winter covers. They do a nice job on boats under 45 feet especially if they have flush decks and no thwarts. Our thwart is an 8-inch riser all around the hull.
Watch out for condensation inside the cabin if you button it up tight. Water happens.