I just have a Sportsman motorcycle cover from Walmart for $20. It's UV protected to protect the plastic from excessive sun damage.
It's water resistant, but NOT waterproof, so if it's going to rain I put a small plastic trash bag over the handlebars to keep water off of the switches and then the bike cover over that.
Of course when it rains while I'm out or at work, the scoot just gets wet! I dry it off when the rain stops.