Here's the half I have been traveling with for years. Will pack to near softball size, dries quickly, easy on and off, no metal to scratch paint, not fully waterproof but 98% of water runs off and does not soak your electricals. Some mornings, the dew is almost as heavy as rain, and this cuts the majority of dew on the bike. I do not have Ultimate seat covers, and with this I have never had my seats retain water. With the supervalk, it drops down to cover most of the backside. Also tourists will not stick their kids on your bike for pictures.
http://www.amazon.com/Dowco-26014-00-Guardian-Traveler-Cover/dp/B000GROHP6These are expensive and intriguing. (like a condom but with no ribs)
http://www.gezagear.com/